The endpoint I mentioned above is simply a WebURL that isn't part of the standard user interface to do advanced functions. You can call that url with a curl command from your computer to set advanced settings not typical for most users. Your desire to use over 300 Kasa plugs + whatever else you are doing on a /22 network is far from any Plug & Play environment. Allot of potential complications come from changing your network to something larger than a class C network or the regular 252 useable hosts. This is prosumer or enterprise type setup so plug and play is kind of out of the window.
If you want to keep it simple plug and play, then you may want to think about breaking the environment up into parts. Then have a hub for each part. Lastly you could possibly use the smartapp Hub Connect to bring it all together on a single hub for management. Ofcourse this is more costly as you need multiple hubs, but it keeps you from having a hub problem and breaking everything. It also helps keeps everything plug and play and simpler. The only non-standard thing in this setup would be making sure you can talk between the hubs and the UDM you have can take care of that. Your netrwork guy should be able to arrange that easily.
Hubitat can easily handle over 340 devices, but that is when you talk about Zwave, Zigbee, Wifi, Lan connected, ect. These setups exist in users homes today. The problem you are having is the limits of networking, networking protocols, and potential pit falls of running connectivity of consumer grade hardware over a enterprise type local area network or LAN.
Please keep in mind the use case you are asking for is very niche. Like .0000001% i would expect. Asking for it to be plug n play isn't really realistic.
