In case someone is interested here is a Cyrillic phonetic keyboard:
Spoiler:
<Patch>
patch_name = `Cyrillic phonetic keyboard (all except Aura One)`
patch_enable = `yes`
# patch_group = `Keyboard alternatives`
#
## Replaces keys on the Extended Latin keypad with Cyrillic alternatives.
##
## Note that after the device boots, the keypad might just show blank squares
## until after the first book has been opened.
## Also note that long-pressing keys on the base keypad will no longer show
## the appropriate list of Extended Latin keys.
#
# Enable uppercase for `ß` (sharp s) key
replace_bytes = 96913E, 00 24, 01 24
# Replace layout sign
find_base_address = `ÉÀÇ`
replace_string = 0000, `ÉÀÇ`, `абв`
# Change keyboard layout
find_base_address = `ý\0\0ÿ\0\0š\0\0ž\0\0`
replace_string = 0030, `à\0`, `я`
replace_string = 0048, `è\0`, `в`
replace_string = 0058, `ì\0`, `е`
replace_string = 005C, `ò\0`, `р`
replace_string = 0060, `ù\0`, `т`
replace_string = 0034, `á\0`, `ъ`
replace_string = 004C, `é\0`, `у`
replace_string = 0064, `í\0`, `и`
replace_string = 0068, `ó\0`, `о`
replace_string = 006C, `ú\0`, `п`
replace_string = 0000, `ý\0`, `ч`
replace_string = 0038, `â\0`, `а`
replace_string = 0050, `ê\0`, `с`
replace_string = 0080, `î\0`, `д`
replace_string = 0070, `ô\0`, `ф`
replace_string = 0088, `û\0`, `г`
replace_string = 003C, `ä\0`, `х`
replace_string = 0054, `ë\0`, `й`
replace_string = 0084, `ï\0`, `к`
replace_string = 0074, `ö\0`, `л`
replace_string = 008C, `ü\0`, `ш`
replace_string = 0004, `ÿ\0`, `щ`
replace_string = 0040, `ã\0`, `з`
replace_string = 0090, `ñ\0`, `ь`
replace_string = 0078, `õ\0`, `ц`
replace_string = 0044, `å\0`, `ж`
replace_string = FFFFFFB4, `ß\0`, `б`
replace_string = 0094, `ç\0`, `н`
replace_string = 0098, `đ\0`, `м`
replace_string = 007C, `ø\0`, `ю`
replace_string = 001C, `ij\0`, `,`
replace_string = 009C, `ł\0`, `.`
</Patch>