You're entirely right. In this case I am going to continue to credit to BAC (and BAEC before April...) as I am not that far off lifetime status (though a lot further than on the 30 December), and it suits my travel patterns well (living in HKG, using the CX F lounges with OWE status, doing plenty of intra-asia CX flights, but also 2xRTW tickets a year with the family - that all combined should keep us all as Golds for a few more years)

Maybe to be explicit for anyone who will come to this thread in the future: When booking a RTW ticket using the AA RTW desk, there is visibility to British Airways on the implied/calculated spend for AA flights, which prior to 30 December 2024 was used for Avios earning, but will, post April 1, also be used for New Tier Point earning of course. The implied spend doesn't however get passed over on for example QR/CX or other tickets on the RTW (as they don't have a data sharing agreement with BA clearly). As Dr. HFH says, you can credit this to non-BAEC/non-AA FFPs and there is no visibility (but they may calculate things differently)

Here is a simple example of what can be seen. Note those very low GBP amounts won't be good for your new Tier Point earnings. In this case:
BOS-LAX / A (Transcontinental). Old TPs == 210, NTPs = 399 (Implied value at 13.3333x == 2800)
MIA-BOS / D (Domestic First). Old TPs == 40, NTPs == 193 (Implied value at 13.3333x == 533)


Example BAEC posting from an AONE ticket using AA flights - note the calculated GBP values

So you may find it better to post any of your AA/BA flights to a different airline rather than "waste" the miles and Avios.