it would be nice to have a variant of Ukrainian keyboard. In such layout the button long press would show a menu with the correspondent Russian letter is missing in Ukrainian alphabet.
Here is the example how it is done in Linux:
A temporary workaround could be to use custom punctuation (
Settings > Text prediction > Custom punctuation) or custom AutoText (
Settings > Text prediction > Custom AutoText). Use the Russian keyboard to enter the characters in the custom punctuation/AutoText and they should be available when using the Ukrainian keyboard.