Re: Travel with 2 small bodies, lens recommendation help
I don't think a lot of people actually do this tbh but I have and I've loved it... I'm still shooting my GX850 with the 42.5/1.7, 75/1.8, or 35-100 f2.8 right alongside my FF kit actually. When I was solely shooting M4/3 I really liked having an UWA on one body (originally the 9-18, then the PL9-18 because I'm an UWA nut, now a Tamron 17-28 on FF) and something like the 45/1.8 or 35-100 f4-5.6 on my other body, with the 20/1.7 always in my bag.
I adore the 75/1.8 and I actually think it's sharp enough that cropping it to match xx-100mm lenses will do fine in a pinch, but I do think it leaves too much of a gap further below the FL range for it to be the only complement to an UWA zoom or even to a normal-ish prime. The Sigma 56/1.4 would be the better choice if you really want a fast short tele prime that can covers lot of use cases.
The Pana 35-100 f4-5.6 may not have the range of some of the other teles btw but it's practically the same size as a prime, eg as small as the 9-18. No other tele zoom can really claim that. I find anything over 100mm to be less useful for everyday shooting (150mm feels like no man's land for me, not long enough for a lot of wildlife or really compressed landscapes), but YMMV.
I think 9-18* + 20/1.7 + 35-100 (or 56/1.4, or the 60/2.8 macro) would make for a heck of a trio in a very very compact package. *(or 9/1.7, but I think the zoom's versatility is key with this kinda kit) They're all small enough that you can have them on a second body and pull said body out quickly or have it hanging anywhere without it being obtrusive, eg I often hang my GX850 with the 75/1.8 off the side of my bag on a Peak Design Capture Clip.
I loved shooting Oly & Pana bodies btw but it can be confusing, Oly UI is a little trickier to get used to, but each has their strengths so once you get used to it their differences can actually be an advantage rather than a hindrance. I would consider just adding a small M4/3 body for the teles alongside your A7C tho, teles are where the biggest size/portability difference often lies.
The Samyang 18/2.8 is the size of the 9/1.7, granted there's no FF UWA as small as the 9-18 tho... Same for the 12-32 pancake. So if that's a big draw then go all in on M4/3, the 14-140 & 14-150 zooms are also hard to compete with size-wise and can be very handy for travel, those might pair better with the 9/1.7 than a tele zoom or even a normal prime would IMO. I love UWA as I said before so an UWA zoom tends to beat staple for me.
So am alternative trio would be something like 9/1.7 + 14-140 II + whatever, vs the earlier trio that has the advantage of being weather sealed while covering a wider range even better and you can still throw the 20/1.7 in and basically bounce between it and the 9/1.7 on your second body. The 14-140 focuses relatively close and you can augment that with a clip on achromat adapter like the Raynox DCR series.