You can't completely. Android is free software, but the driver blobs are not, and are often known to have security flaws.

It's not something that I am OK with. It's a compromise.

I am excited for the librem phone[0], but I can't afford something like that at the moment.

[0]https://puri.sm/shop/librem-5/

Run a custom ROM and monitor network connections. Don't trust, verify.