If I were going to do this myself I'd just make a script. As you say, sounds very tedious to do this by hand. Then again you're reading each one. I'm surprised you don't charge more for this. Then again, I'm surprised that people feel the need to do this at all. LOL!
I currently dont offer removal of posts for this reason. I dont see a way to automate this without possible deleting crucial posts and Id have to charge a very higher amount for doing it manually. I am certainly open for offers, but deleting e.g. my ~3.5k posts would take me roughly the 4 weeks I have before deleting the random data.
On the other hand if you want to remove your own posts you probably would not read them all again and could easily automate it.
i think with a bot, you can filter the posts you want to mantain...
for example...
if you want to maintain last post
https://bitcointalksearch.org/topic/m.11009687
you need to pass to your bot/script topicID/msdID to skip deleting.
or better, still i think msgID it is univoc inside bitcoin talk forum...
so could be enough.