After studying the game for a week and working on a ‘seed effect’ generator, this is the seed with the most beneficial effects that I could find.

I've spent the last six days working on a generator that lists all the benefits and interactions of seeds with each other, and I think I've found a seed that has all effects that I consider good and none of the bad ones. It has Celebration (for way more things like Sword Shrines and more NPCs at start), Drunk (for both ores), a ton of shimmer on one side of the world (from Drunk+celebration), the ability to teleport by changing team (making it easy to get to the side with water), no Dungeon Guardian (Dungeon is hidden but you can shimmer into it), world is see-through and has more tunnels so you need to dig less, lots of rainbow items including dye, rainbow cursor, and more Rainbow Guns, portal guns in chests, tons of floating islands, pumpkins that spawn on start for easy pumpkin armor, bigger underground houses, and more!

I left in traps (so rainbow boulders can spawn and traps can be sold for money or used for mob farms), spider caves (for early hardmode weapons and armor), and didn't do the infection-eliminating seed as it makes it harder to play.

The seed and a complete list of its effects are below:

2.2.1.9.abandoned manors|beam me up|hocus pocus|jagged rocks|jingle all the way|rainbow road|royale with cheese|the care bears movie|we dont even test for that|xray vision|1732327512

• There are six spawn points in the world, one for each team. Single players may change teams, and changing teams teleports the player to that team's spawn point.
• Much bigger abandoned houses generate.
• Teleporters can spawn connecting different locations underground, even if there are no solid blocks nearby.
• The entire world is coated with Illuminant coating.
• There are extra Floating Islands. Some may spawn without houses. There is a 1 in 3 chance that they will be made of Star Cloud.
• The amount of extra Floating Islands from the care bears movie is reduced.
• Portal Guns have a chance to spawn in Chests.
• The world is constantly set to Halloween mode.
• The world is constantly set to Christmas mode.
• A party starts when the game begins. By default, this party has a guide named Andrew, a Steampunker named Whitney, a Princess named Yorai, a Party Girl named Amanda, and a Party Bunny. Any changes to this are listed below.
• The Party Girl named Amanda is replaced by a Painter named Jim.
• The Party Bunny is replace by a Clumsy Slime called Slimefors.
• Pumpkins spawn on world generation.
• Half of the world generates with Shimmer replacing water.
• The Dungeon is buried but no tree is visible above it.
• There is no Dungeon Guardian, and Skeletron does not have to be fought to enter the Dungeon.
• Palm trees drop triple wood and ash trees drop triple wood.
• Slimefors is invincible, with maximum health, 99999 defense, and inability to be attacked.
• Snow (Otherworldly) will play during world generation, the background will be the Hallow, and heart-shaped clouds will appear.
• Sand is painted rainbow and sand walls are painted black.
• Extra gel can drop from slimes.
• Ash Wood items are weaker.
• Players spawn on the beach.
• Both world evils spawn, and both Demon Altars and Crimson altars appear throughout the world. Both evil Biome Chests appear in the Dungeon.
• Only 2/3 as many Bee Hives spawn.
• Living Mahogany Trees are much more common.
• Both types of ore spawn in each category.
• Wooden Spikes generate 3 layers deep instead of 2.
• Enemies are more likely to spawn in Bee Hives. Bee Hives are larger and contain more Larvae.
• Marble and Granite biomes switch generation styles.
• Tunnels from the Jungle surface to the Underground Jungle do not spawn.
• Moon Lord Legs can spawn in chests.
• Red Potion can spawn in chests.
• The moon is a smiley face during world generation.
• Otherworld Music is unlocked.
• Music defaults to Otherworldly Music.
• Constant falling stars spawn during world generation.
• The Jungle Temple is much larger.
• Items in chests spawn with only good prefixes.
• The Jungle spawns slightly closer to the middle of the world.
• Some Rainbow Boulders spawn.
• The Dungeon Guardian won't spawn until 70 ft below the surface.
• Ocean caves always spawn.
• Extra caves, mostly vertical, are generated throughout the world.
• There are more Living Trees than usual.
• The maximum number of Enchanted Sword Shrines is doubled, but the probability for one shrine to spawn is halved, except the first one which is guaranteed.
• Two extra items are added to the Travelling Merchant's shop.
• Princess sells the Slime Staff, Flask of Party, and Heart Lantern, as well as Sandstorm in a Bottle while in the Desert and Terragrim during Blood Moon. After pirates are defeated, she sells different pirate items during different moon phases.
• Any generated trees and mushrooms are painted random colors.
• Fairies are more likely to be pink.
• Palm trees drop a bit of extra wood.
• Mimics are more common and Jungle Mimics may spawn in the Jungle.
• Golden slimes may spawn.
• Blood Moons and Birthday Parties are more common.
• Fireflies and butterflies are more likely.
• Many bosses are twice as small.
• The Wizard Hat provides an additional summon slot.
• Dev Armor sets are 2x as likely to drop.
• Halloween- and Christmas-themed hearts and mana stars can drop all year round.
• Fallen stars are painted pink.
• The Tax Collector gives twice as much money.
• The Rod of Discord is more likely to drop.
• The player bursts into confetti upon death.
• Respawn time is cut in half.
• Fairies are more likely to spawn underground.
• Underground houses have a 50% chance to be converted to rainbow brick.
• Players make Don't Starve sounds when killed.
• Coin rain is more likely.
• 50% more gem caves spawn.
• Gem Trees are much more common.
• Half of all moss caves are Neon Moss, and twice as many moss caves spawn.
• More Bouncy Boulders spawn.
• The Jungle Temple has lime green tiles.
• The Jungle Temple has dark green walls.
• All trees are Yellow Willow Trees or Sakura Trees.
• The Dungeon is painted a somewhat random color dependent on the base color.
• The Shimmer spawns closer to the surface.
• Life crystals are 20% more prevalent.
• Water Walking Boots are more likely to spawn in Water Chests.
• 5 times as many The Dirtiest Blocks spawn.
• A pyramid spawns in the middle of the Desert.
• Pyramid loot always contains the Pharaoh set.
• Shimmer can spawn outside of the main Aether.
• More buried chests spawn.
• The chances for a Lava Charm to be generated in a Chest is increased from 1/20 (5%) to 1/15 (6.67%).
• More ore is generated when altars are smashed.
• Rainbow glowsticks replace glowsticks in chests.
• Rainbow glowsticks replace glowsticks in chests.
• All variations of Rainbow Dye (Normal, Intense, Living and Midnight) and Meowmere Minecarts can be found in Chests.
• Each Boulder has a 1/4 (25%) chance to be replaced with a Rainbow Boulder.
• Some Underground Cabins are made of Helium Moss Bricks.
• Rainbow Flares can be found in chests that contain a Flare Gun.
• Rainbow Cursors can be found in Surface Chests.
• Rainbow Campfires replace normal campfires in Campsite structures. The Campfires on certain Dungeon designs remain unchanged.
• Rainbow Torches replace the Demon Torches in Ruined Houses.
• Rainbow Bricks replace every block a waterfall flows over, making all waterfalls rainbow.
• Caves and the Dungeon have Helium Moss and Disco Balls in them.
• Rainbow Slime Banners replace all (decorative) banners in the world.
• Each of the Biome Chests contain a Rainbow Gun in addition to their normal loot. The Hallowed Chest still contains a single Rainbow Gun.
• Rare clouds are more likely.
• The logo rotates during world generation.
• Random numbers display at the bottom of the screen during world generation, except when placing traps.
• Underground Cabins can spawn with Slime Furniture.
• The Snow and Jungle biomes spawn on the same side of the world.
• The beach is as large as possible.
• Pyramids are more likely.
• The middle portion of the Underworld is lava, with the outer edges having ruined houses.
• Surface tunnels can generate over a wider portion of the surface.
• World evils avoid the beach more.
• Snow sky islands and desert sky islands may spawn.