As Dietz says, you can use MIR with Synchron products - I quite often do - but there are various caveats. You need the sound as dry as possible, to turn off Dry Signal Handling, etc. It depends on the sound you're after - again, if you're after that big concert hall/scoring stage sound it's pointless, you're better off with the baked-in Synchron Stage sound, but there's a lot of other really interesting stuff you can do.
One issue I have with the Synchron stuff is the panning. Because I'm not making classical music, a lot of the time I don't want a conventional classical orchestral layout. For instance, with the kind of stuff I do, having both violin sections on the same side of the stereo field can cause balance problems and I often want them either side, with violas and/or celli in the middle. I'd be interested in any thoughts on how best to go about that when using the actual Synchron sound rather than MIR.
A lot of the time I'm using a combination of Synchron and VI and/or Synchron-ised libraries. Both of the latter always go via MIR, with the former it's horses for courses.
Mac Mini M2 16Gb RAM 500Gb int. SSD 2Tb ext. SSD Pro Tools/Mixbus An awful lot of VI, Synchron-ised and Synchron libraries, amongst others. VSL user since 2003.