Regarding LH - it's a bit more complex than what you described. Flying other *A carriers do earn status points, but you do need a minimum threshold of flights on LH Group aircraft for status - that much is correct. As for TK - let's not forget that their IT and logistics are a nightmare to have to deal with.
Looking at the LH changes a bit closer, you're right. Whose brilliant idea was it to have a type of points called Qualifying Points, but not make it the only points needed to... you know qualify. Why not just have one type of points and say "at least 50% needs to be on Lufthansa Group flights". Anyway, it's still out for me.
Doing a bit more research, I'm leaning towards SQ KrisFlyer even though I expect to rarely or ever take an SQ flight (at least for work). It seems to have a straightforward miles-based system with no requirement to actually fly SQ and with few or no (?) booking classes on partners ineligible for earning. Any reason that would be a dumb choice?