As I mentioned in Post 2 above, the nodes you're using (RT-AX56Us) are not a good match to the main RT-AX88U. They are degrading your overall network performance.
Your 3,000 SqFt home/area is much smaller than the 5,000 SqFt (nominal) area the RT-AX88U can cover if it is appropriately located.
The RT-AX88U was a great router when it was introduced. The RT-AX86U was noticeably superior. Be sure you are not using LAN Ports 5-8 on the router. If you are, get/use an 8-Port switch instead.
The location of the nodes is important too. The location, relative to the main router and the area(s) you want to light up with WiFi, should be strategically sought and tested for. Do not use a WiFi 'app' to determine this location. Only experimenting will give it to you.
As an overview, do not place routers/nodes above/below each other. Do not use any more nodes than absolutely necessary. Determine if nodes are required at all by first properly locating the main router and adjusting its Control Channels and antennae orientation as necessary.
How to Connect an AiMesh Node
AiMesh Ideal Placement
See more tips at the following links below.
After creating the checklists two weeks ago to save the pages with settings, I've made all the screenshots (I think at least) of all the relevant guides written by @L&LD and the very useful tip by @netware5. The only thing is, those guides are great, except when you'll be offline at least for a...
www.snbforums.com
www.snbforums.com
Buying a new AX class router, and placing it correctly in your home as needed, will greatly benefit your ISP speeds and main router.
The nodes you're using today are worth only using in Media Bridge mode (see link above for details) for your far away (from the main router), wired-capable devices.
Your most bang for the buck upgrade today would be the RT-AX88U Pro, to replace your current router as the main router of your home.
Test it and dial it in fully before attempting to add any nodes (and the only one worth adding would be the current router you're using as the main one, right now).
For such a small area (275 metres square) and assuming you're not living in a brick/concrete walled/floored home, you don't need the finicky/gimmicky 'tri-band', 3-radio routers to greatly improve your network performance today.
Before you buy anything new though, unplug or turn off those nodes. Unplug all Ethernet cables from ports 5-8. Find the best location and antennae orientation for your main router (note that even a few inches can make a world of difference at distant corners of the areas you want coverage in). Also, test for the best Control Channels for your environment and re-adjust the location/orientation of the router as needed. You may find you already have enough equipment to fix the issues you've expressed above.