FYI, there is some more information about RSSI and LQI in ZHA docs (including uow to interpret the values and why you should not look at them during normal operation) → https://www.home-assistant.io/integrations/zha#zigbee-network-visualization-in-zha-ui → https://www.home-assistant.io/integrations/zha#how-to-interpret-rssi-and-lqi-values
As mentioned above, suggest just ignoring RSSI and LQI values for now, instead only focus on spending time optimizing your physical setup (including adding a few “know good” Zigbee Router devices), so I higly recommend starting by following the best practices and hopefully everything will then magically sort itself out so you will not need to do any troubleshooting - > Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Regarding RCP Multi-PAN/multi-protocol; I did just write a long rant yesterday trying to explain the disadvantages of using RCP Multi-PAN/multi-protocol on a single radio SoC versus using separate dedicated radio adapters for each protocol, you can find it here in this other post → Zigbee Utilization 90%, normal for 130 devices? - #15 by Hedda
Summary; modern Zigbee and Thread USB dongles are relatively inexpensive so suggest following the KISS principle and simply buy separate dedicated radio adapters for each RF protocol. Check out this Zigbee buyer’s guide → Zigbee buyer's guide (just buy separate radios with one dedicated Zigbee Coordinator radio adapter and one or more dedicated Thread Border Routers, noting that the are different radio type recommendations if using Zigbee2MQTT instead of Home Assistant’s built-in ZHA integration).
Anyway, yes there are downsides with RCP Multi-PAN and it is still also experimental, but it should not affect range (as that is meant to be solved by having a proper Zigbee network mesh), but running in RCP Multi-PAN mode with both Zigbee and Thread protocols active at the same time does seriously affect the total amount of devices your can connect. There are many variables but I do not advise connecting more than 30 devices if using RCP Multi-PAN mode. PS: They mentioned more about that during the Home Assistant 2024.2 Release Party that the RCP Multi-PAN effort will be put on the backburner for now → https://www.youtube.com/watch?v=8-YwXkgD3CY
Regardless if you use a separate dedicated Zigbee Coordinator adapter or not, I highly recommend that you also read and follow these best practice tips for the best possible experience → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage
Other than putting your Zigbee Coordiantor adapter on a long USB extension cable to a USB 2.0 port or hub (and not a USB 3.x/4.x ports), you will need to add many mains-powered Zigbee Router devices.
Another important thing to keep in mind is to never power off Zigbee Router devices as they always need to be available or devices connected to them will lose their connection, and even if that is just temporarily it will make for an unstable Zigbee network. A common mistake there is to connect Zigbee lightbulbs/lights that act as Zigbee Router devices to dumb switches that can be switched on an off. So not connect such Zigbee Router lightbulbs to dumb switches. If you can connect Zigbee lightbulbs to dumb switches then you need to buy Zigbee lightbulbs that are not Zigbee Router devices, like example those from Sengled.
For a larger home I recommend buying a few (suggest at least three or more) dedicated Zigbee Router devices, meaning products that were designed to be nothing other than always connected Zigbee Router.
So for a quick no-effort fix regarding adding Zigbee Router devices I normally just recommend that people new to Zigbee who have big houses and large areas or buildings with dense building materials make it easy for themselves and just buy a bunch of “IKEA Trådfri Signal Repeater” devices from the start to get them a good mesh network backbone as a baseline. This is because while not as strong as these Zigbee USB dongles with an external antenna, the “IKEA Trådfri Signal Repeater” comes with good firmware by default and is very inexpensive, so you can make up for them not having the highest performance by simply buying more of them, (and they are still more powerful than almost all other commercial Zigbee products that are not designed to be a dedicated Zigbee Router device).
https://www.google.com/search?q=IKEA+Tradfri+Signal+Repeater
However, if you want to get the best setup then the best AND most cost-effective Zigbee Router that money can buy today is just to convert a few of ITead’s “Sonoff Zigbee 3.0 USB Dongle Plus V2” (model “ZBDongle-E” based on Silicon Labs EFR32MG21) into dedicated Zigbee signal repeater / Zigbee range extenders by flashing them with Zigbee Router firmware and powering them with a simply USB-chargers. While maybe not the prettiest solution to look at, if you make sure they are permanently powered then joining/pairing three or more to your Zigbee network and spreading them around in your home will create an extremely stable backbone in your Zigbee network.