Upgrading G85/GX85 MFT to S5 or A7C

I've been using the G85 and GX85 for a couple of years now for video. I love the M43 system. Particularly the size and IBIS in these cameras. I have a gimbal but mainly handhold as I find the IBIS good enough. I do occasional promo videos for my work but also use them for family video which I like to make look good.

I'm looking to switch to full frame as I'm finding the photo side of things sub par with M43. I'd like to keep the advantages of a smaller body and good IBIS so I'm looking at either the Panasonic S5 or the Sony A7C for my next camera

Things that matter to me: Half descent IBIS: I'd like to be able to take half descent video handheld without micro jitter. I find the G85 is enough for this.

Size:I'm amazed at the size of the A7C. It seems like the perfect size with the 28-60 kit lens for the pocket. But I could slap on more hefty glass if needed.

AF: I'm amazed at Sony's AF and I think it'll be great for filming kids running about. Something that the G85 and possible AT won't be good at.

Things I'm not fussed about:

10bit video Viewfinder Recording limits

I'm swaying towards the A7C due to the size and AF but I'm worried the IBIS won't be as good as the G85.

The S5 is slightly bigger but the AF concerns me. Plus the lens selection right now looks a bit thin and expensive. I know Sony FE lenses are also pricey but at least there's a descent 2nd hand market already.

I know the A7C isn't out yet but has a similar IBIS to the A7iii. Does anyone have experience with the IBIS and handheld stabilisation with that system?