The controller never required Windows for anything as far as I know. Linux has always been the best way to run it, and remains so to this day. There is standalone mode in the mobile app for home users, yes. It provides a much more limited feature set though, and I'd argue it's mostly for home users.I bought my first (and only) Ubiquiti device almost a decade ago, they've improved in some ways (the controller software doesn't require a Windows VM to run a Java application just to configure a single AP,) but also added a bunch of cloud stuff with proven questionable security, plus the setup we have at the office just flakes out randomly and requires one or more hard reboots of every Ubiquiti device in order to get the wifi working again. I would argue they've had lots of time and haven't made any net progress, if anything their products are getting worse.
The cloud stuff is completely optional. I disable it on my controller at home that is managing two different sites. If we're thinking about the same cloud compromise, it turned out to be a disgruntled employee who used his own credentials to steal data in order to extort them. I still keep it disabled though as it doesn't provide any benefit for my use case.
We have a UniFi WiFi setup at the office and I haven't had any problems there with lots of people constantly having Teams meetings and similar. Something is clearly broken in the setup you have at your office. Not sure what though, but it's not a normal thing. I've had good experiences with the two AC generations and the U7 generation. The U7 series certainly had some firmware issues initially (and still clearly do for some), but I was thankfully not affected by them.
I should be said that I haven't used their switches or routers/gateways at home so I haven't used that part of the management interface.