You’re right, buffer size settings is not available for HQPlayer configuration, sorry.
What does your cpu usage do? HQ Player can be pretty intensive. Not sure what your new dante card does to the mix, but could be that adding it all up no longer fits. In which case you could also try to switch your hqplayer filter to a -2s variant which is less cpu intensive.
If that doesn;t help you can consider posting in the support forum so one of the Roon support staff will pick up your q.