I have the 28-70 STM, great lens, I had the EF 24-70 2.8 L II and swapped it immediately for less then 200€ difference, nice upgrade, as I don't really use wide focals much, so 24mm isn't missed; a matching 70-180 STM or whatever would certainly be interesting to swap my ancient (but still hot and sharp) EF 70-200 2.8 L classic (non-IS).
But, doing weddings, and 70mm being not enough, I crave the Tamron 35-150 because I can have a single lens that covers both for 24/28-70 and 70-180/200, and instead of having two bodies always on me, I can just have one (35mm on the widest end is enough for 95% of my shots), and the second body would be ready in the bag, only when needed, arguably with a Tamron 20-40 2.8 or something similar (small, cheap, bright, as the use would be much reduced). Today if wideangle is really needed I have the 16 STM, small and inexpensive, but it's too wide, even a fixed 18/20mm prime would be enough for me, in Canon there's nothing cheap (20 VCM is overkill and overprice, it's a lens that would shoot no more then 50/100 pics per year, so 300/350€ is the max I would invest in it), and the old EF 20 f2.8 that would be perfect isn't any good wide open.
So today I have to remove 70-200 from the second body to accommodate the 16 STM in emergency, while in a possible Sony future (...or if Canon finally allows FF third party lenses on RF...) I would just pick up the second body, took those 5/10pics needed, and store it back for good in the bag.
I would buy an used A7IV, and that's why I'm on the verge of moving to Sony since almost two years, but I haven't done yet, because I'm waiting for the A7V to be released, so the A7IV price drops and the used market is flooded by cameras form people wanting to upgrade.
A7III or A7C feels not enough advanced in the AF system for being my primary camera (could certainly be the second, today I work with primary R6 and secondary RP), the A7S's have too low mpx, and the A7R's have too high mpx, I'm happy to move on from 20mpx (which is my resolution since 5D II in 2010 and passing thru the 6D...yes, time to have some more after 15yrs) but 42/60mpx from A7R's are not needed, the 33mpx of the A7IV feels like a perfect sweet spot. And yes, I know that R6 III would have the exact same sweet spot, but my primary concern is having one lens that does it all, and the R6 III doesn't solve the issue, like it doesn't solve it the RF 24-105 2.8 L which is too expensive, and also too short on the long end to be the do-it-all lens that I'd like to have.
Now, Canon semi-new course with lenses like 28-70 STM, 16-28 STM and the possibile 2.8 tele zoom companion, and the coming 45 f1.2 are certainly helping me to stay for the moment, probably without the 28-70 STM I would already have gone in the last year, so something better is happening, but I still don't feel it's enough for me.