Read that the new upgraded Su-30SM2 will share engines, radars and other devices from the Su-35S making them more compatible and simplify logistics / repairs. They will in effect become the 2-seat version of the Su-35. Plans are that over time all the Su-30SM's will be brought up to SM2 standards.
Initially the Su-35s systems were untested and immature, but were also related to things developed for the Su-57... now that they have operational experience with the Su-35, including in Syria, then it just makes sense to fit that same equipment to all Flanker type fighter bombers.
The Su-34 stuff is probably different enough to warrant its own upgrade but some things for the Su-35 could certainly be used but it will need its own upgrades of other unique things like the radar for example.
This means despite having various versions of upgraded Su-27s, Su-30s, Su-35s, and Su-34s, they are going to end up with essentially a single (35) and two seat (30) fighter/medium strike aircraft and a medium strike (34), plus the stealth pair of Su-57 and S-70.