Chinese company, little info about them out there. Website is very generic with little info about who they are. Like you saw, there are very few reviews and even those are just promo types without any real data. Amazon reviews are lousy. I would avoid it. There’s a reason the larger brands cost more - they have actual support and engineering in place. I’d opt for reliability over price.
Hard to know which one would bring you instant relief.
I would move out on #2 in terms of getting some wiring around. If you’re adding cable, do ethernet (unless you have a need for the coax for some reason). Ultimately, to improve your network in general, you want to wire around the house and provide ethernet to devices that can use it (saving the wifi bandwidth for those that can’t, and convenience items). Also, you can add wifi APs (access points) to improve your wifi coverage for the devices that use it.
Not yet - do your cable planning and upgrades - or at least integrate that planning when buying hardware.
Yes. It’s a good idea for separation if needed, but still a good idea if they are colocated.
Ethernet. Do ethernet. No extenders, you want access points. They will perform better in general and often have more capacity for number of devices they can each support.
No mesh. Mesh is for people who can’t or won’t wire ethernet. Mesh is wirelessly uplinking access points, so it’s wifi over wifi. Ethernet (or MoCA, which leverages coax into near-ethernet speeds) is far preferable. Mesh is not a better network, it’s just for getting around wiring things.
If you are not confident in your ability to crimp the coax, you could have someone with proper tools and connectors re-do it.
Ridiculously reliable is how I describe my UniFi setup - I’ve been running it since 2019 and I could not be happier with it. Updating to a UDM Pro next week from a USG-Pro-4/Cloud Key Gen2+.
I dragged my feet on updates, there were issues a few years ago. However, each update went fine for all devices when I was pushed along, so I’ve gained confidence in them and have been impressed with the improvements in things. I am a former sysadmin and didn’t want to have to have my old job as a new hobby and so far, so good.