I have 1-5 works in dlsite ... I've never seen 6-8 before, and dlsite has recently added author registration and search, but it doesn't support older works. Such tankoubon can be a way to find works by authors I like that I didn't know about, but it's hardly worth buying them again...
The dlsite search functions do not really work... the best site that has good searches are getchu or DMM but both do not contact a lots of doujinshi.. So, I usually go search if the author's Twitter / IG, pixie post etc.
Search functions did not set up probably is one thing, the other side is most authors won't put all of their books on one site. Most will spread up their creation into multiplets sites to sell. I don't understand is a tradition or what but that does cost us lots more time to search their works.
the other site that is mostly used are: dlsite https://www.dlsite.com/maniax/ DMM https://www.dmm.co.jp/dc/doujin/ melonbooks https://www.melonbooks.co.jp/book/list.php?category_id=71 toranoana https://ec.toranoana.jp/tora_r/ec/?