I don’t fly neatly as much as you by the sounds of it but I do maintain AA and BA status. The AA platinum gets me bumped on domestics quite often and the BA Gold lets me use the Flagship Lounges even when crediting to AA.

I tend to credit US domestic flights in Y to AA and premium cabin flights to BA. I like to have a good stock of AAdvantage miles since redemption rates are very often better and always as soon as there is a connecting flight in your journey.

To answer your question on tier points post 1500, your next bonus is at 2500 which will get you a GUF2 voucher. This can upgrade up to 2 people on a complete (BA-metal) itinerary. This can be used on a redemption or, importantly, via a travel agent on a cash booking to upgrade into revenue buckets without needing award inventory. At 3500, you get 2 GUF1 vouchers, the same but each for 1 person. Other options come at each 1000 tier points. GGL and the CCR will come at 5000+.