Your chosen lens - the 21mm Color-Skopar - is an awesome lens, and you're creating some really cool stuff with it this month. That said, I don't blame you - I don't think I could shoot all-21-all-the-time for a whole month. An option could be when you find a good subject, shoot it with your 35, 90, and 21mm lenses and focus on emphasizing each FL's strengths. That could give you a diversified portfolio for the month and not drive you batty doing the ultrawide-only thing.
It may just be that an ultra-wide (21mm is borderline in that category in my eyes) *you can't really get close to subjects with* can be a burden. I've shot way wider lenses (I'm not sure about everything, but I remember the Laowa 7.5mm f/2 fondly) and found the experience to be rather refreshing, even uplifting. But: Whatever lens I've chosen so far in that regard, they always had the ability to get really close, so I could actually fill the frame with a subject if I wanted.
Remember: Carrying *additional* camera/lens combos isn't against the rules if you need a bit of respite. But depending on how you "work", the SiF combo should always come first whenever you're shooting. It pays to tune into whatever subject matter works best for *that* setup. If you find something else, you can always break out the alternative for a spell.
I sometimes need gear that will stand up to the weather (hence the G1X III), and a sufficiently versatile zoom on longer walks in order not to have to let too many opportunities pass. But I'm really lucky this time around - I can make things work with my SiF setup alone most of the time. At f/2.8, the lens balances focus point sharpness with pleasing bokeh very nicely, at f/5.6, it's a nice enough landscape/cityscape/street lens, from f/4 to f/8 it works for most subjects. And wide open, it has the signature dreaminess of a vintage lens. I have to say that I consider this a very lucky pick - and a fortunate one: I wasn't sure whether the lens was a keeper. It would have been a real pity to move it on without giving it another chance.
That said, I may try its predecessor in another "Single in" - that lens is *way more* capricious, but I liked its character a lot back when I was shooting it on its native Sony E mount (on an A6000). And I have a dumb E to Z adapter ... We'll see.
At the moment, I feel tempted by the ongoing discussion to do the next challenge (Single in September 2023) with a super-wide; maybe the Panasonic 9mm f/1.7, but more likely something more exotic. I have just the thing - the Laowa 9mm f/5.6 for M mount. Yep, full frame ... I'm not sure I should do something *that* crazy, though.
M.