Thank you both for your replies - they are appreciated. In response to the bits I've picked out (although all was appreciated):
Yes, certainly "master of none". I've come to the guitar through a variety of instruments, most of which I ended up being "very intermediate" at! It took me a long time to accept I was never going to be brilliant at any one of them (although I did sing professionally when I was younger, which I suppose is as near as I got to being "good" at anything). However, it means that, now I'm over that psychological hump, whatever progress I make with whatever I'm playing, I'm happy with - but I do have to make some progress, I think, keep things fresh, or else I end up moving onto something else anyway.
The reason for saying all that isn't outrageous boasting (I promise!), but to acknowledge the 100% of popular opinion so far that says "go with the acoustic". I'm happy to be led. Either choice would probably be "fresh" but the acoustic (this one, in case anyone is interested - https://www.andertons.co.uk/alvarez-artist-parlor-acoustic-electric-non-cutaway-eq-tuner/ ) does offer a bigger departure from what I've been doing and more scope for new technique than a new LP as you've observed.
I particularly wanted to say that I appreciated your comments on "rhythm playing". To be frank, musically, I've ended up doing things in completely the wrong order. Again, intended humbly, I grew up in a classical background and for various reasons relating to early education, my theory knowledge is good and was in place long before I was ever good (or intermediate!) at playing anything. But on the flip side, much more significantly, the repertoire of music that I know for guitar is very, very limited (worship songs, mostly), even in terms of listening (I'm trying to remedy this) and I find lead guitarists / shredders / anyone who can move around the fretboard at speed downright intimidating!
Being brutally honest, I lack both idiomatic sense for any particular genre and much of the experience of listening to bands, knowing their songs and trying to copy / learn them and so on. As such, I've sort of defaulted to trying to learn rhythm playing and chord progressions as it's the closest thing to traditional harmony you can do on a guitar and probably the thing that matches up best with my previous experience. No doubt my constant looping around satisfying progressions irritates my family and the neighbours! But I have been working slowly through Rhythm Guitar 365 by Troy Nelson and to be honest, it was the exercises marked "Acoustic Guitar" in that book that put the idea for acquiring an acoustic into my head in the first place. Whatever it is I do, it's enough for a few songs at church (not my thing either TBH, but it my wife's and I love to play) every now and again.
So in a sense I have an advantage as far as attempting your suggestion of trying chord substitutions, which I will try to incorporate: at least I'll understand the instructions and should be able to apply them sensibly! I'm going to have to look up "drop-two chords", but the rest of your pointers I think I can follow and although some of them are within what I already try to do, I'll use your suggesting them as an opportunity to refocus on technique and less on playing personally-pleasing, but probably not developmental things for the foreseeable. Sometimes, I guess, you need someone else to tell you to do what you know deep down you should be doing but let slide.
ATB.