Just to chip in and hopefully not spoil your thread I am in a similar situation and have been for quite a while.
I went down the mesh rabbit hole a few years back and bought the best I could afford which was the netgear orbi rbk50.
I went the whole hog and got the router with 2 satellites, like you I have VM 350 and the super hub is in modem mode.
Again like you I have had mixed results, the closer you are to the main router the better the speed then the further you move away it drops off.
I suffer from clients not connecting to the best/strongest signal which infuriates but I think I can sort that out by separating my ssid's but to be honest I haven't motivated myself enough to do it because it will probably take me an age and end in tears, I'm not very good with this sort of thing.
I have over the years tried loads of different wireless routers/setups trying to get decent coverage and I've come to the conclusion that wifi is pretty useless in older bricks & mortar houses.
No matter what the sales blurb says if you have a house (in my case a bungalow) with solid walls don't expect miracles.
I am contemplating selling my orbi system and going down the route of separate wifi AP in virtually every room using POE.
Again the upheaval and complexity scares me but I think it's the only way to get consistently fast wifi in every room of your house.
It would also pay to add some hard wired access points too so again it's not for the faint hearted, maybe I should just pay someone to do it for me
Anyway good luck on your quest
PS just to add, this mesh wifi system is far and away the best system/setup I've had and I'm guessing in houses without solid walls everywhere is probably perfect.