I was wondering if there are any repositories or wikis of formal testing techniques - test design ideas, like pairwise or boundary. I couldn't find one so far.
The Wikipedia Software Testing category sounds like the sort of thing you're after.
I agree your thought of a wiki for testing techniques could be a valuable idea for software testers.
The best testing technique repositories that I am aware of are of the "dead tree" variety.
Lee Copeland's "A Practitioner's Guide to Software Test Design" is the best book on the topic in my view. Available only in book format.
Also well worth checking out (particularly since he has generously made pdf versions of it available for free) is Torbjörn Ryber's "Essential Software Test Design." (Available in hardcover book or free pdf download).
Lastly, while not technically "testing techniques," Michael Hunter's "You are Not Done Yet" has a long list of excellent quick ideas to consider testing before you declare that you're finished.
Hope these help.
You may want to check out the Software Testing Club Wiki - the checklists and heuristics sections are particularly relevant, given what you've said in comments.
For resources on test design specifically, check out the Test Design course materials available at testingeducation.org. Once you've done that, check out the other BBST materials. There is enough there to keep you busy for a couple of years, assuming that you also have a day job!