It is very good. But again I have a couple of remarks. Usually, if C has a closed aperture, then 3 Э as well, but here you can make differences for 3 Э акщь С, but they differ too much from each other. Д Л are also very different, but this can be forgiven here, although they usually have the same shape. And Д Щ Ц has too short descenders.
I would also make the У a little narrower. And the short over Й is too unusual for this design, it's better to make it straight with a small upward strokes at the ends.

I wonder if the latin lowercase would be possible to do. Spacing on letters like ч and ш night need checking because they appear to a bit wider than other letters

Already better, but still there is something to fix. Instead of З, to make it lighter,, you can put the number 3, since you have already made such a з in lowercase. Э is still not square, this unique slash is useless to it (and for б). The letter б should be done like here. Lowercase у ф needs descenders, they can be lower than the цщд have, and it is better for the ovals р and ф to be as high as o.
The kratka (over й) protrusions are also too narrow; in constructivism, the stem width should not change. And the kratka is too close to й, also the stick is too close in capital ы, and in lower case wrong left part. And BWM is right.

б

Nice can you update this font called Perosto/Prosto?

Congrats, Master!

Finish line. My last comments. In my б the lower part is larger, the upper descender should not be less than in Latin b. The same for ф, it is narrower than ш, you need to make ш narrower or ф wider. Э still shouldn't repeat З. And I expect you to correct ч ш ы.

"З" and "Э" are identical.