Seems like you’re on Windows? If not, and you’re on Mac, try the AU plugin instead.
On Windows…
You might try running it through a bridge.
A few options existing…
Free Options (That I know of)
vst3shell
Kushview Element
Commercial Options (That I know of)
Plogue’s bidule
Bluecat’s Patchwork
I’ve never tried the Synfire Pro host your post is about specifically, but I have enjoyed the benefits of using vst3shell and bidule for better VST3 compatibility in various hosts. In fact, I use them with hosts that don’t support native hosting of VST3 plugins at all to get HALion and Grove Agent going (Band in a Box/Real Band/Real Tracks, Sibelius, and Finale).
vst3shell is hosted as VST2, and provides a bridge for sub-hosting VST3 plugins. It seems pretty solid with HALion/Sonic and Groove Agent to me so far, though I notice it only supplies one (where it should be 4) MIDI input ports for full HALion 7 (the rest seems to work fine though).
bidule can run as either vst2, or vst3, and can then host a variety of plugin types from there, including CLAP I think. Bidule can be tweaked a bit in the system registry to establish some base defaults for specific hosts that aren’t quite getting set up as desired/expected through host and plugin initialization protocols. I.E. I can force it to have Dorico always begin bidule instances with a 4 channel input, 32 channel output configuration by default as far as the host knows, and then divert things where I want in bidule and the sub-hosted plugins from there.
I’ve also tried Kushview briefly, and it seems to work as both VST2 or VST3, but I can’t figure out for the life of me how to get it to simply pass program changes to hosted plugins instead of changing Kushview presets. If you don’t need to change patches in HALion using MIDI Program Change, it should be pretty solid. There may well be a way to have it pass PC events direct to plugins instead of managing Kushview’s own patchbay, I simply have not figured it out yet if it can!
Patchwork is one I have yet to try. I don’t know a thing about it.