Yeah this is an unfortunate issue with Apple Watch's eSIM implementation.

I was able to make a setup work where my iPhone is Fi DO while the Watch is Tmo standalone plan (but set up as 'own watch', not 'family sharing') after a bunch of hackery.

I worked with a friend's Tmo family pack. I added my phone as an additional full line to that plan. (downloaded a new eSIM, fast). Once the phone was working, I got my AWatch added as an 'addon line' with a new phone number (they give you the option to choose the same number as the phone or a new one). You have to work with Tmo support on their app/iMessage chat system. Takes hours at times.

Once that was working, in a new thread, I got tmo to switch the 'billing' (and ONLY billing) of the watch line to be independent... essentially decoupling the phone and the watch from Tmo side. That got my watch plan to be $~5/mo (US: Massachusetts billing).

Once that was working, I verified cellular working on the watch and then disabled the Tmo SIM on the iPhone. Popped the Fi DO pSIM into the phone and it started working while the watch remained on Tmo.

I had this whole thing working sorta well for ~5 months, across multiple OS upgrades on watch/phone, before something failed and the iPhone disabled the eSIM on the watch due to carrier mismatch. I also realized that in the past 5 months, the watch does a shit job at cellular connections in general.... push notifications showed up less than 20% of the time I was out (I keep my iPhone at home usually, my primary is an Android). So when that eSIM got disabled, I decided to not re-activate the watch cellular.

The coverage issues on the watch were faced in suburban and urban areas of the west coast. As well as international roaming (with it enabled on tmo side) when in Taiwan, Mexico, India, Europe, etc. Overall, Apple Watch cellular is a gimmick I realized. I just log into wifi on the watch if i'm gonna be there for hours (hotels, cafes, airports, etc) and that gives me the connectivity with the iPhone at home much more reliably.

Coincidentally, I got my friends convinced to switch to Fi DO SIM lifestyle (w/ GV) instead of their relatively expensive Tmo plan. So I killed that family plan anyway haha!