How low have you managed to go on the shutter speed when handholding your A7m3? It's a pretty general question and doesn't take into account all different factors, but I'm curious how good the IBIS is.
The IBIS isn't nearly as good as M43's implementation. I try to stay above 1/40th if possible. Back when I shot with Olympus bodies I could go down to 1/5th-1/10th depending on the lens and subject I was shooting.
Back in manual focus film days (so obviously unstabilised) I managed some sharp images at 1/15s handholding a 50mm, and no doubt failed with some at 1/250s.
With digital it's often practical to have a number of efforts when pushing the boundaries, so a 1/5s exposure should be achievable with stabilisation, a little persistence & reasonable technique. With good technique & a little luck 1s is probably realistic - I've certainly seen it done with an Olympus.
Given the other factors it's very difficult to determine how effective stabilisation is. You really have to test your own limits, to get an idea of what you can manage, and for meaningful results you need at least 10 repeats to overcome the variable aspects.
Obviously there is a difference in the form factors of these two combinations. The A7iii combo is quite a bit larger and heavier than the A6400 combo, The A6400 seemed easier to keep still as I shot...
Here are three shots using the A7iii and Tamron lens at 75mm at approximately the same shutter speeds as above...