

Once updates end in October, Windows 10 won’t be secure and probably shouldn’t be allowed to connect to the internet. It will still continue to function though. You can buy a year of extended support for Windows 10 for $30, but it’s unclear whether there will be support after that.
Probably best to start switching to something else, or plan to run that PC offline.
Custom Android ROMs based on AOSP, which is the open source parts of Android. I used to use LineageOS back in the day.
Before buying a phone, make sure that it has an unlockable bootloader and a custom ROM of a recent Android version. Note that some phones have multiple versions for different regions and some might not be unlockable, while others are. For example many Samsung international phones are unlockable, but none of the USA models are.
Note that many commercial apps like Netflix, Pokémon Go, bank apps, etc. will refuse to run if your bootloader is unlocked. It also permanently trips Samsung’s Knox.