The question for enviroian is where do they plan on traveling the most? Is it major cities or small towns?

If the former, then I would keep Hilton Diamond as your backup and shift to Hyatt for primary. Since you are EXP on AA, you should be able to get Explorist immediately and quickly level up to Globalist; that's how I switched over from Bonvoy in 2019 although with LT Plat on Marriott, it is a perfect backup. Since Hyatt has a small footprint, you need a Hilton/Marriott secondary program.

If it is smaller towns, then Bonvoy does make sense. That said, I would still split my stays with Hyatt as getting anything above Platinum on Marriott is a waste of time. Platinum on Marriott gets you breakfast, wifi, 4 PM checkouts etc. all the key stuff needed. Titanium and Ambassador are higher status levels, but the benefits are YMMV with regards to being tangible. Some hotels, like the Sheraton Grand Sydney rolled out the red carpet, while others couldn't be bothered.