Honestly I agree with others here. It's really not as bad as you're making it seem.
The problem is, it is as bad.
Unlike other in-game grinds, this particular one is a really stupid IRL Perception check.
You either know exact cleansing ranges AND are savvy enough with in-game coordinates to utilize that knowledge AND capable of opening 100% Surface+below map AND capable of spotting that goldern single tile on minimap (because patrolling whole Large world is... let's say, for people with too much free time) which is about to ruin everything - or you're not.
From my point of view, the only solution under current set of circumstances is:
1. Right from the spawn grab infamous Cacti Armor + Sword.
2. Grab some Sunflowers along the way. I believe, 8 is a bare minimum (2 per evil biome spot, up to 2 evil biome spots, each biome might get splitted by something or have a cave you'd like to protect or have other types of potential spreading spots).
3. Nosedive into nearest hole and hope that you'll get both enough Iron for Minecart tracks (rough estimation for Large world: well over 16k Tracks, woohoo!) and enough Bestiary completion to get Molecart.
4. Reveal WHOLE Surface, look for lowestmost surface spot (including bottoms of lakes but excluding Crimson/Corruption chasms). Place Sunflowers on the way to and fro.
5. Start making a... how do you call it in English... sanitized zone? Two rows of Minecart tracks slightly below lowestmost spot of Surface, Molecart digs 4 tiles high tunnel so 8 tiles total. Pray that you aint getting extremely bad Deserts which might need you to dig thousands of sand blocks on the way.
6. Only after you've finished securing your Surface you can actually start playing the game as it meaned to be played.
7. Eventually, Dryad would arrive. Waste surplus cash on cleansing powder - and 100% clean the surface + finish sanitized zone in already-present evil biomes.
8.
HAAARDCOOOOOOORE HARDMODE!
9. Contain with extra Sunflowers or purge with powders surface Good & Evil biomes as needed ASAP.
10. Continue playing as it was meaned to be played, knowing that you won't have to drop the world you've invested so much into just because of some bad game mechanics which punishes the player when it's too darn late to fix anything.
One step left or right from this sequence - and you either end up with too much surface junk to purge (while being under pressure already because pirates and the like don't ask for consent, and biomes are spreading) or (if you DO have too much free time to waste) are eventually forced to participate in unfun "spot the spot" minigame... worldwide.
I'm not asking to remove the mechanics completely.
Since it backfires in a long run (and is very easy to miss a point of no return unless you're preparing for it from Day 0) and as it's really pacebreaking to deal with its consequences,
Give Us Convenient Tool To Deal With It Cthulhu Dammit !!