So few months ago I purchased a 3-pk Deco X68/AX3600 that was offered at Costco for my girlfriends house. Thus far things have been amazing, connection and speeds is heaps and loads better than the ac1200 Google Wifi (Nest) pucks that she had previously but that has seemingly always been my experience with TP Link products.

Stuff that I have noticed recently has got me wondering if I need to setup her house (midwest two story with fully furnished basement (approximately 3k sqft all together) to better cover the house. I have found that really only the furthest side of the house from the main unit is problematic for connection. This is one of the kids rooms and where the xbox x lives. I have tested multiple times there and can regularly pull anywhere from 150-300mbps down and 50-150mbps up but there are random occasions each day where it suddenly drops to 20-30mbps. This is the only room this happens in and again is the furthest from main node as well as secondary node (only using 2 of the 3).

1. Would I get better, more consistent speeds by having all three nodes connected?
2. The main node is on the main floor at one end of the house nearest to the modem (Google Fiber) where the line comes into the house. This seems to adequately cover the main floor, backyard and basement with speeds up to 500mbps (on gig fiber service but highly doubt getting more than 800mbps ever). Unfortunately, this is the only place this can be placed. Would adding a second node at the other end of the house (30+ft away) in the kitchen or possibly in the garage create a better coverage net?
3. The second node is on the top floor, middle(ish) of the floorplan in a spare workout room. My gf has commented recently that when she is on a phone call (call over wifi) that the person on the other end says she breaks up until she opens the door to that room. These are interior doors so nothing should be blocking the signal significantly enough to cause this behavior. Testing with WiFi spectrum testing app on my Lenovo ThinkPad does not show much of any change in signal strength when opening/closing the door to that room or to the bedroom. This one has me a bit baffled.
4. Is there a way to designate which devices connect to which node? There is usually a maximum of 14-16 devices connected to the network between phones, watches, laptops, PS5/XBOX, tvs, security cameras, etc. but aren't pulling data (no active connection based on the Deco app). I would like to be able to ensure that certain items attach to the main floor unit and other attach to the secondary (and/or third) node.

I have checked for firmware updates and keep the Deco App up to date but haven't seen anything with regards to the last question. Thanks for any help you can provide.