Question A WiFi/Router recommendation

[Small_Tarzan]

So, I've got a 75m2 flat with thick walls between each of the rooms. Right now I've got a set of 2 old Ubiquiti Amplify Instant routers and one Ubiquiti LR 6.
However, since Amplify and Ubiquiti aren't the same "subbrand", they won't integrate into the unify console, hence are difficult to control. I've tried changing the whole setup to the TP-Link Deco X20 with terrible results. A $300 Mesh system that wouldn't connect at first, doesn't have an ethernet backbone for each module, and is generally slow to operate.
So, I'm asking, I have a $300 budget to improve the WiFi in this flat. Ubiquiti is overpriced in this country (20% more expensive), and is only WiFi 5, so at the moment it's not an option.
Would any of the commercial solutions help? Or should I just buckle up and get a Dream Machine/Router or something? Or would you recommend something else altogether? Perhaps a $300 router instead?
Thank you!

 

[bill001g]

So do you have ethernet already it is confusing from your comments

There is no magic solution to solve the walls absorbing radio energy. There are no mesh system that will punch a signal through a wall just because you pay lots of money for it. Almost all radio unit transmit at the legal maximum allowed by the government....even $20 routers do.

If you have ethernet cable you should be able to cobble together something that works with all the equipment you have. You just need to treat the remote units a dumb AP. They don't need to do anything more than convert the wifi to ethernet. You don't really even need to manage them once you have them set up.

Everything is one network when you use AP, mesh systems are not doing anything different. There also is no such thing as seamless roaming the end device not the network decides on what to connect to and they are not real smart. Best to have as little overlap in wifi signals as possible to give the device the best chance.