Now, I'm looking for a true car replacement for commuting that is also a lightweight (comparatively) trail bike. At the same time, as I don't accumulate 'stuff', I'm looking for the last bike I will ever need to own. I want to invest in that platform, and haven't figured out what compromises I'll be okay with yet..
I mean, I'd just look for a hardtail with whatever motor you want and clearance for reasonable tires and go to town. I think you're overthinking this. Some of your wants are in conflict with each other (full suspension is needlessly complex overkill for a commuter, and the geometry that makes for a good trail bike isn't ideal for riding on pavement or commuting). Belt drives are low maintenance, but then you want to saddle a commuter with suspension at both ends that will need at least annual servicing.
Jack-of-all-trades bikes tend to be mediocre at a bunch of things. It really makes more sense to get an actual mountainbike for mountainbiking and a commuter for commuting.
I'm not a mud thrasher, I ride carefully. Why? I had to repair people's bikes everyday with the stupid things they would do to them, not understanding mechanics. I'd see how poorly seasoned mechanics would end up slapping together repairs, because they got burnt out dealing with the carelessness of people. I know that I am the only one I trust to do any repairs and builds. Any bike I get, I always end up rebuilding and fixing all the errors. When I ride a belt off road, if I do.. I for certain won't be the type of person to thrash up a gravel cloud or rooster tail of mud because I care for my machine and it is an extension of my body. It's the responsibility that comes from doing all the work yourself. So, I'm looking for ideal technologies put together in a bike that I've always wanted.. and I want that with serious torque and power in a mid-drive for late nights that I need to get home and am exhausted from work.
Riding carefully doesn't make that much difference when you get to real MTB trails. Maybe you just mean, like, dirt roads? MTBs break. They get exposed to dirt and water and bounced on rocks, and you will definitely crash eventually if you spend any time on singletrack. I do my own maintenance too, but I expect to break things because I like to ride difficult terrain. I don't lose sleep over it, I just don't spend big bucks on derailleurs or brake rotors or rims.
I had a road bike that I built from the ground up that weighed a solid 27 lb., but I'd go for rides with dudes who had carbon Cervelo's and Pinarello's and Willier's. Many times I'd end up besting them because my wheels and drivetrain were optimized for micro friction. That kind of optimization doesn't 'happen', it takes months of work to build up, and if you thrash on a bike, you destroy many micro regions that can't be repaired, even on an $8k bike (10 years ago, that was about the most expensive carbon road bike.) So, I'm just hyper aware of these things. Categories and customer warnings really don't apply as much when you are in-tune with the machine you ride.
I... don't really know how to parse this. Your average road bike is ridiculously efficient, and any semi-fit roadie is going to be cruising at a speed where wind resistance starts dwarfing drivetrain losses and rolling resistance. Theres a reason that pros doing TT stuff are waaaaaaay more concerned with aero than they are with fancy low friction drivetrain mods (or even low bike weight).