It’s a feature not a bug. A feature to maximize profits for of the vendors that create your hardware that is.
For vendors a so called vendor lock-in is a great way to make money. Unfortunately they have an incentive to make their stuff not work well with other stuff because that may mean that you’ll buy more of their stuff.
Now, in some cases they have to implement compatibility, because otherwise they’d have trouble getting customers int he first place, but in it’s often obvious that playing well with others isn’t a priority.
Hence solutions like home assistant rely on shoddy interfaces and that shows in the form of issues. Besides, it’s a volunteer driven application. You can’t expect the people who make it in their free time solve all the problems for you.
Edit: Basically, you will have to solve these issues step by step. Afaik they are all solvable. And of course you might want to rely on hardware that’s more standardized. E.g. onvif cameras. That’s not necessarily less of a hassle than with ring etc. but it’s cheaper.
Have you tried different servers (you can select country and city in the app)? Have you tried it on different days of the week at different times?
That would be my starting to find out where the bottle-neck is. Under optimal conditions Mullvad should get very close to your 400mps (the protocol has some overhead, but in a speed test with “full” packages that’s only about five percent of your bandwidth). Of course they can’t guarantee optimal conditions on all servers all the time, but it’s also not rare for them to get close.
The distance to the Mullvad server doesn’t matter much when it comes to bandwidth. Hence, unless you’re gaming and actually need a short ping, you can try servers that are further away.
If you don’t get anything higher than 150Mb/s at any time with any server, we’ll indeed have to look for technical issues on your side.