Dunno what languages you talk/use, so going by what I do:

• Pretty light and good for European languages from my experience, there's Simple Keyboard: https://f-droid.org/packages/rkr.simplekeyboard.inputmethod/

• Heliboard's also great for Korean, if you learn it at some point.

• Mozc is technically Google's project, but is on F-Droid and is pretty good for Japanese writing, so kinda of a gray area, I guess: https://f-droid.org/packages/org.mozc.android.inputmethod.japanese/

• Fcitx5-Android is pretty good for Asian languages too, specially if your prefer phonetic keyboards instead of Mozc and Heliboard's visual approach: https://github.com/fcitx5-android/fcitx5-android