It is distinctly non-trivial. Likely part of the reason why it remains possible. Chasing down bugs is the unavoidable result, but to a geek it's manageable. MicroG works very well once sorted. It is however astonishing how much processing power Play Services gobbles up. A de-googlefied phone lasts 30-40% longer on a charge. Once done, it works like a breeze. 3-4 year old flagships Androids are titans when de-googled and with custom roms.
This looks promosing - "New company called eelo will give you de-Google-ized Android so that you can keep your privacy"
If the phone can get LineageOS, then it is fairly easy - just don't install GApps when flashing LineageOS to the phone. Alas, phones that are thus open are getting rarer and rarer :( .