I used the files and followed the steps from this tutorial:
https://www.teamandroid.com/2018/07/...stom-firmware/
Since asking this question i managed to install chrome, facebook and messenger which all work fine and also youtube, which doesn’t work, saying it needs google play services, which are not supported by my device (??)
I’m very pleased with how the 6 year old tablet works until now but i’d like to get google play store to work to be able to install some apps for which i didn’t find an apk to download...