I have shot MFT and FE both for years. Typically, the FE is for landscape on a tripod and the MFT for everything else. The big difference between older FE bodies and any Olympus/OM body (all the way up to the A7Riv) is that MFT has much better IBIS. That means it works better in low light unless you are shooting action.
More recently, I got an A7CR and that has quite useful IBIS but still nowhere near an OM1 or OM3.
Your A7iii has 24Mpix and a weak AA filter. With a decent MFT lens, you ought to get very slightly sharper images with the OM3. Probably depends very much on which lens you compare with which.
That leaves subject isolation and dynamic range as the advantages of your A7iii. If f2.8 on the A7iii gives enough subject separation, there are several f1.4 MFT primes.
You have a big choice of UWA, normal and short tele zooms including some that are up to mk iii.
I am a RAW shooter, so the jpeg tricks of the OM3 are of limited interest, but they are very significant if you shoot jpeg. The computational photography works really well because of the fast sensor readout (on my OM1). HDR, LiveND and Handheld HiRes are all useful and compensate for DR to some extent. The OM3 has in-camera AI noise reduction. It’s not quite as good as DeepPrime, but pretty close.
If you are a RAW shooter, set a Custom UniWB, use neutral JPEG settings, shoot to histogram and maximise exposure. You need to do some test shooting to find the jpeg to RAW exposure headroom. The free Workspace uses AI noise reduction on OM3 RAWs and does a good job with detail in handheld hi-res pseudo-RAWs.
Subject detection on the OM3 is going to be a big positive surprise. Afterwards, you will want an A7Cii or maybe not!
TL

R I’d always choose my OM1 or OM5 over an A7iii for travelling to Madeira. Choosing between my A7CR and an OM3 would be harder. I have plenty of lenses, so that’s not the issue.
Andrew