Help with Replacement Network setup

Hey,

I'm really after some help here. (UK Based - EE, 1gbs down, 300mps up)
Current situation

ONT > EE Router > ASUS RT-AC68U
Backboned on this is:

x1: ASUS RT-AC68U
x2: ZenWiFi AX
in AIMesh mode (ethernet)
and switches in between are unmanaged switches

i have a 24 port (only 8 ports used) in the garage where the Main Asus connects.

the issue is, since this was brought many years ago, at least the AC68U's are failing. With constant reboots

to name a few things running:

Plex Server, Webserver (small web apps), File Server (photos), Wifi Camera system, Tablets, Phones, Apple Tv, Laptops, Sensors etc.. along them lines

My question and why im asking for help from you. I want to replace these 4 devices with a better reliable system that can handle the throughput. im thinking Unfi equipment.

But i do not know where to start on what to replace with.
I was think the UCG-Ultra
But what AP's and what switch(s) if any

Budget roughly £600/£700 - but im somewhat flexible.

Can anyone recommend the replacement equipment for this setup

Thanks in advance and help :)