OpenKeychain is great for keys on Android. It's FOSS and available on FDroid. +1 for using a self hosted Nextcloud instance to keep things in sync.

I keep my keys on a Yubikey. I've not really had a need to use the GPG portion of this on Android, but it works fine on Linux and MacOS.

I used this guide: https://github.com/drduh/YubiKey-Guide

keep them in my NC and just import them to thunderbird

That's an interesting dilemma when you bring up Android. I have always considered android device as a hardware compromised device and that it shouldn't be used for highly confidential data to an extent that you might be using PGP/GPG for.

But you could have all of your PGP/GPG centrally managed on a Linux system with android device having it's own unique keypair that is signed by your root PGP/GPG keypair on your Linux system. As for software for managing GPG/PGP on Linux system, I just simply use KGPG which does the job plenty well. If you have to use PGP/GPG on Android Phone, then I recommend sticking with f-droid repository for PGP/GPG key management app, not Google Play Store.

OpenKeychain Source Code

OpenKeychain Package on F-droid

Few use-cases for GPG/PGP on android is encrypting email or chat, but application integration is limited to select few software like K-9 Mail or Conversations.

--Edited to add--

Why the heck did server spam duplicates of my comments? :(