I get that a hybrid is attractive because of the flexibility, but still the change is a strange decision. EVs are simpler to maintain than ICEs, but a hybrid is more complex, it adds the possible EV problems atop possible ICE problems.
Maybe keep the trucks as much they are now, just the essential changes to replace the engine? There's plenty of space on those huge trucks.
I think it's still simpler, actually. IME the most complicated part of an ICE vehicle is the power delivery system. Transmissions are nightmares to work on. Making that all-electric and just using an engine to generate power significantly simplifies the system. I'm not a mechanic though, so take my word with a grain of salt.
My understanding is that going to hybrid actually allowed Toyota to significantly simplify their transmissions relative to ICE vehicles, even without going full EV.
The planetary gear "eCVT" systems that Toyota and Ford use in many models are mechanically a lot simpler than a traditional automatic or sequential manual transmission. Few moving parts and no clutches at all. I don't know what the long term reliability of those drivetrains is is but I wouldn't be surprised if it's measurably measurably better than a traditional transmission + engine. There's a long educational video from Weber State University that gives a good walkthrough of what's going on in those things.
It turns out that it reduces possible ICE problems since you use the engine less often while the electric powertrain doesn’t add enough new problems to matter, so the result is an improvement in reliability.
Maybe keep the trucks as much they are now, just the essential changes to replace the engine? There's plenty of space on those huge trucks.