I feel like the answer is a combo of: boy shorts (or some other sort of modest-coverage bottom), and just coming to terms with the body you're living in.
At least that's how it's going for me so far!
I have a couple pairs of these in different colors:
https://athleta.gap.com/browse/product.do?pid=657741012&cid=1026482&pcid=1026482&vid=1&nav=expmore%3Aswim%3Acategories%3Aswim%20bottoms&cpos=2&cexp=1501&kcid=CategoryIDs%3D1026482&cvar=11273&ctype=Listing&cpid=res22022805612954841280800#pdp-page-content
I wear them when I'm not wearing board shorts. I don't feel like I am fooling anyone in them but I also don't feel completely naked in public.
I've been meaning to try these:
https://itsnowcool.com/collections/bikini-bottoms/products/the-boy-short-rust
Even when I was younger I didn't like wearing tiny bikinis - I relate strongly to Tobias Fünke's never nude tendencies - but, yeah, at 48 I feel like these sorts of bottoms are good,
Also trying to learn to accept and even love your middle aged body! (I know; I know! But seriously.)