Can I live long-term in Chiang Mai by doing visa runs with a German or Russian passport?

Realistically if you wish to live in Thailand long-term, tourist visas don't cut the ice. You're going to have to look at strategies (work, education, elite) which allow you to remain on a long-term basis

Nobody will have experience of this as it's simply not doable for an extended period of time

Currently 45 days plus 30 days extension. The 45 days runs out next year and returns to 30 plus 30.

Visa runs are not a way of staying long term in Thailand. You’ll get away with one or two, perhaps more but at some point they’ll pull you aside and grill you or even deny entry.

You can do two land border runs visa exempt in a calendar year. No technical restriction by air other than above.

German passport holders and Russian passport holders both get 45 days on entry with no visa. German passport you can extend for 30 days. The Russian one can’t.

It sounds like you're not asking about visa runs but asking about border bounces with visa exempt entries. A visa run is when you go to a Thai embassy in another country and apply for a visa, then return to Thailand.

Regardless of the details of your question, living long term in Thailand without a long term visa is not going to work. You will get questioned and denied entry after a couple of times.

Germans currently receive 45 days upon entry without a visa, called visa exempt, until March 31st when it returns to 30 days on entry. This can be extended for 30 days at the local immigration office, giving 75 days total (or 60 after March 31st). You can only do visa exempt entry at a land border 2 times per calendar year.

As mentioned above, after doing this 2-3 times in a row, they are going to start asking you a lot of questions and basically assume you are working illegally in Thailand and deny you entry.

Russians do not qualify for visa exempt. Russians can enter on bi-lateral agreement and will receive 45 days. They cannot apply for an extension since it's not the same as visa exempt.