I have a naive suggestion. If I were in OP shoes, my focus would be to choose a carrier for my long haul TAPC trips based on schedule and cost. Schedule means routing w.r.t. # of stops, departure/arrival timing etc. I'd then decide which FFP I should credit the mileage earnings to.

My thinking is that (1) as a lower level elite, the benefits are limited anyway (except if you are tall and traveling in coach then E+ is a meaningful benefit from UA); (2) OP's goal is to accumulate miles over long term, however the best rewarding FFP today could devalue by the time OP has sufficient miles for a meaningful redemption. Now if OP could get to mid-tier (or willing to MR for it), then *A gold is a very sweet spot...