I am in need of a separate degoogled phone for some things that require high level of privacy (nothing illegal).

I have 2 phones that I can use. One of them is my business phone (it has my business number, apps, data and that sort of stuff) which is now running an OS with all the Google spyware because it's necessary for the apps to work. I can reinstall everything on the second phone and use the first one as the secure device since it supports everything I need. The problem is that it has some issues on vanilla ROMs that I don't really want to deal with and the reinstalling will take a lot of time.

The issue with the second phone is that it is rocking an old MTK chip and rooting instructions are let's say a bit beyond my ability to understand. I still want to use it without Google if possible though. So can I degoogle its stock ROM with ADB or something? And is it worth trying or there will still be some vulnerabilities?

EDIT: to clear some possible misunderstandings, the reason of why I need a separate secure phone is that I am forced to use a very invasive proprietary app that I'd prefer just keeping on a separate device instead of trying to limit its spyware abilities with firewalls and that kind of stuff. I don't trust the last solution much. Also I can't use it in a VM because I need it to always be accessible wherever I am and yk carrying a PC is not an option