The bottom line is that can take a lot of bikes and miles to figure out what's best for a particular person and task.
Yes, it’s part of the fun and games, in many ways it’s a hobby a little separate from cycling. It’s not every cyclist's hobby, some just buy a bike and ride it. I’ve not been satisfied with every bike I’ve chosen, my first Rohloff bike for example, a Thorn Raven, wasn’t a favourite. The more I ride the easier the choices get, though it took me three bikes over five years to get my camping bike right, a role the Mercury’s predecessor fulfilled in a way my Merc doesn’t.
If anyone were to criticise my decisions, I’d tell them where to stuff it, it’s no one’s business but my own. Likewise your choices, I’d have done that transformation differently and ended up with what on paper is a bike equally suitable for the purpose and better suited to the hub, but that’s comment no criticism.
I’m not commenting to what you’ve done, but the way you’re presenting it.
In 2020 I bought a Mercury thinking it would be ideal for my needs but after doing two supported tours last year where the Mercury appeared overbuilt and overloaded compared to the other bikes
Anytime you write something like that I’m going to point out, as I have above, that your Mercury was overbuilt compared to most other Mercurys. Otherwise any casual observer will likely read into it something that isn’t true. You’ve come round to the idea that a heavy bike with big tyres and all the trimmings doesn’t suit you for fast and light touring, I doubt anyone is going to disagree with you. What you haven’t done is discover if a Mercury could be suitable for your use, only that your build wasn’t, that would likely have been the case with any steel Sports Touring frame, including the one you’ve replaced it with. It wasn’t just me pointing that out at beginning of this process, you rejected the idea on the basis that the aim was two bike for different purposes, what changed your mind?