Being comfortable with your camera is always a good start, but having a good grip of the basic techniques will allow you to get the best out of pretty much any camera.
What genres do you like to take and where do you feel the results aren’t what you had hoped for? It could well just be a tweak in technique that cures it.
I honestly don’t think jumping to an A7 mk3 will automatically improve your photography. The resolution may be a bit higher, but you still have to guide the camera by using the correct settings for the desired shot. It really is down to technique and experience that allows you to create the shots you have in your mind. I knew exactly how the shot of Newhaven Harbour would come out on Friday before I’d even arrived assuming the weather was ok. Thankfully it was so no surprises. I could get the same shot on most camera bodies that give me control.
You mentioned astro photography, will you be using a telescope? If yes it’d probably be a good idea to join some facebook groups or watch Youtube videos to get a feel for what gear they use. From what I have seen I don’t think the body matters too much as they tend to stack the images to increase detail so no worries about using MFT system.
There are only 2 areas where I struggle with the OM-D M1 II
Indoor / low light / high-ish ISO shots
High dynamic-range shots (think sunrise/sunsets) where the sensor… or my technique (?) …isn’t able to keep pace
Both are well-trodden pitfalls with the smaller sensor, though on every other count I am still delighted with the system
However, a Sony A7 / A7R would cure those two ills without breaking sweat, I don’t expect it to improve my photography, as such, just cure remove those 2 weak spots (while possibly adding a few new 'uns.)
I’ve got an A7RIII landing this evening (hopefully) for a nice price, and will be getting a Tamron 28-75 lens over the weekend.
Plan is to try both out in equal situations and see how much I miss the ‘goodies’ that Olympus brings to the party vs the much better low-light / high-DR performance of the Sony