@pranav @fosstodon @switchingsocial
Maybe take a look at the XDA-Thread of your tablet model and scroll trough the ROM section. I even found an Android Oreo ROM for my Xiaomi MiPad1 (not the most widespread tablet model).
Watch out in the ROM features for 'signature spoofing'. You can easily run MicroG with this, to be more independent from Google, if the Fdroid Store isn't enough for you or you relay an other Google Framework features to run certain app
Free replacements for the Android operating system are come by if you have a specific device in mind. You have to root your device, install a new ROM, etc. And many devices require many nonfree blobs for your system to operate.
If you want a mostly free operating system for Android, LineageOS is your best bet. Replicant is the fully free fork, but it is very unlikely to work on your device.
Good luck rooting! Please say how it turns out.
Fosstodon is an English speaking Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.