For that, you want/need an FF body and lens. Yes, crop lenses are occasionally shorter BUT once you're at a 180° field of view (an 8mm fisheye on a full frame/35mm film body), you're pretty much at the limit.
You can get 180Deg AoV on crop... just.... it takes a Sigma 4.5mm Fish, which I believe is still the only 180 full-round for APS-C, and a chunk load of dosh for likely use one will ever have, but still... at around the £500 mark, it still came in a less than going Full-Frame, to get 180 Deg AoV and Full round pictures; even though you can pick up 'budget' fish for full frame for under £200.. all-be it manual focus... but then WHY they made the Siggy 4.5 Auto-Focus is still something of an mystery to me! With a closest focus distance of oooh... next to nothing, and hyperfocal before arms length, even at f2.8 I would have to try really REALLY hard to get anything oof-ed! Vasaline perhaps? But you cant put a filter on the front.... maybe stuffing the filter draw with bubble-wrap? I dunno!
Other Siggy in the bag is the 8-16, 'rectilinear', which again claims, at least when I bought mine, to be the widest UWA for APS-C with a 115Deg AoV, equivalent to approx 12mm on full-frame, and and tending into the murky depths of fishing, even there. (The Fish eye for my film cameras, that prompted this perversion to chase the wide side in the first place! is 12mm, and delivers the cropped circle image)
So you can go as wide on APS-C as on Full-Frame, and there isn't a lot of price incentive to go one way or t'other really.... maybe if you are looking very hard to justify the switch to FF, then it offers some excuse, and 'maybe' some slightly wider choice of lenses, but, its hard to say that you 'must' go full-frame to shoot that wide; you don't .
ANYWAY Back on topic; OP has now admitted that their desire for a second lens, is pretty much just because they have an interchangeable lens mount, and want to be able to use it!
In days of yore, cameras seldom had 'easily' interchangeable lenses; they had a lens, and that was it! IF you swapped camera... you by default got a new lens to go with it, and it wasn't 'such' a long time ago, with the popularity of zoom lenses, that folk suggested that having just a 2x zoom, say 35-70, that covered 'standard' angle, and some mild wide and mild tele, which oh-so often never came off the camera, rather made the interchangeable lens mount of system cameras rather redundant.... in fact, it was something Olympus recognised as early as the late 1980's, with I think it was the IS range of 'all in one' super-zoom SLR's that pioneered modern 'Bridge' cameras....
As for future-proofing? Good luck on that front... I don't think you ever really can, the future, is by nature, uncertain! And does it really matter? You buy a camera to take pictures; if it does, then job jobbed. If they make a new camera in a years time... well, has the one you got stopped being able to take pictures? Should STILL do the job.
If in a years time, you decide that the 'new' camera is an absolute must have for some reason; then you will go buy it... whether that means swapping all the lenses and accessories you have acquired to go with it, and buying over to suit this new must have super-widget, then so be it.... if not, you wont.... and hoping or trying to find some 'economy' in the middle scavenging some utility from what you already have, is just that, meddling in the middle scavenging, and 'hope'... and making the matter more complicated and difficult than needs, just for the sake of.
As said, once upon a time, you bought a camera, it had a lens, and that was it! End of.. no choice, no argument and no pontificating or indecision! Pays your money and take your chances, but live in the here and now, not some elusive future that may never happen!