Polish written in etymological cyrillic orthography would likely appearto be much more similar, than it really is. That is one proposition, how it should look like:Polish would be the closest example to hand; I suspect if both languages were transliterated, with Polish not written in the conventions the Poles have developed but a new one developed to be easiest for English readers to sound out as accurately as possible, and Russian transliterated in the same format, the two would look very similar, as close as English is to Dutch, perhaps as close as standard English to Lowland Scottish! That's my impression, that Poles and Russians differ mainly due to deeply divergent cultural allegiances but linguistically and presumably ethnically were once the same people pretty much. So, insofar as Polish orthography is functional and logical for Poles, with the suitable modifications they have developed, it ought to work pretty well for Russian too. Of course, I have no idea whether Polish conventions are as clunky and archaic and perhaps even as chaotic as modern English is, and Poles learn to read and write with a lot of rote memorization the way we are forced to, or if it is scientifically honed to a lean machine comparable to Italian or Spanish orthography. If I overestimate connections between Polish and Russian, the latter might mean a need for different fine honed adaption of Russian.
http://steen.free.fr/cyrpol/index.html
Although I don't see need to use archaic "Yus" letters. Modern Polish does not have nasal vowels anymore. For former Polish vowels (in modern Polish it is oral vowel + nasal glide) I'd use Э, Е, O, Ё followed by letter Ң (used in orthograpies of Turcic languages) to represent nasal element.
Taraskievica with few modifications also could be easy adopted for Polish.
https://en.wikipedia.org/wiki/Taraškievica
Correspondence between spoken and written Polish is comparable to Spanish (although worse than in Serbo-Croatian or Turkish), lots of digraphs and trigraphs make Polish words appear to be longer than they really are and lots of fricatives and affricates make impression, that it is some "szczszz" hissing/hushing, but it fits the language quite well.I have a terrible time trying to understand what sounds conventionally written Polish is trying to make, it is as bad as trying to read modern conventional writing of Welsh or Gaelic for me! So I'm just as glad Russian did not go Latin since a good orthography for that language probably would involve conventions very misleading to an English reader, unless English were indeed the original template. And barring a reversal of the Alt-Tudor/Late Rurikid-early Romanov political alliance in the inverse challenge thread, there is no reason for such a thing. And a reversal would be even more far fetched and precious and absurd than the proposed adoption of adapted Cyrillic by the English; I gave a bunch of reasons for the English to do that that would not work the other way at all.