So you could backhaul them together and save space on the 5G band for the devices or if cable isn't possible then a Tri band mesh will do the trick.
I don't think such a thing exists but teltonika gets quite close. I'm also interested in such a thing.
Throwing all your traffic through the router is to be avoided if you can. Especially if it's an ISP provided cheapo. Get yourself a nice switch with extra APs if needed. TP link Omada is great. Think unifi but sensibly priced.
Wow that's a lot of devices for one ISP router. First thing I would do is swap that for a commercial grade one with an AP. Something like tplink Omada or unifi if you prefer. I know it's a chew but run that eth cable to your wired devices. Long term it's really worth it since 2Gbit+ is becoming the new thing.
If your dead set on Unifi go nuts. But at least check out Omada.
Don't be making expensive descions on a complete network stack just because it looks pretty.
It COULD work but the final results would depend on your MoBo and the drivers playing nicely.
Ethernet is definitely the way to go if you can. If not a pair of outdoor bridges will get you connected albeit much slower.
TP link Omada stuff is great. It's like unifi but cheaper and minus the tribal fanbois.
A C is perfectly fine considering the speed of your connection. Use the MoCa with a switch upstairs. Connect your pc and the Mini hub to that. You should see a nice improvement with both devices.
Who's your ISP ?
Look I'm an independent enginner doing remote or on site support. I see all sorts in people's history etc. Do I care No. Let's face it everyone pees poos and screws.
Ain't no shame we're all the same.
Go for 3. It's a pain to get out of CGNAT on EE
Change the APN to 3internet on whatever board / modem you use and that should see you right for failover. I'm not sure you to config that on your particular HW though.