East Slavic languages only differ by the choice of i/y letters and Belarusian's short u, Ukrainian yi and ye letters (and technically the apostrophe). You can have a combined Ukrainian and Russian alphabet keyboard. I have it on Heliboard

Bulgarian uses the er golyam/hard sign as a vowel, which it used to be in every Slavic language in the past. Macedonian is mutually intelligible, but uses an alphabet similar to BCS Cyrillic.