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One we like is the unfortunately named “FunWhole” on Amazon.

Though to be honest I think these are increasing in price on Amazon lately :(

But there are tons of other sellers and many of the sets seem to get good reviews

the subreddit r/lepin (named after a knockoff brand) is all about lego-compatible brick sets and has a wiki with known good brands.

Gobricks are very high quality generics, mould king and cada have good technic sets and electronics

I used to be very pleased with the Polish "Cobi" range. They make a lot of military models, which were great for my dad, who has mobility issues that made traditional model-building difficult.

But then we bought one of their biggest sets-- a Maus tank-- and he complained it was missing parts and unmanageable. I suspect thousands of gray-on-gray parts might have exceeded some unknown challenge threshold.