"Proprietary syntax highlighting system" is absolutely a deal-killer here. That would be the thing that would make me stand up and say "avoid this like the plague." Figure out how to do this without doing that. Obsidian did.
Thanks for this. Not sure I understand what the concern is. It's proprietary in the sense that it is unlike what you would have seen elsewhere. But it uses plain natural language (ie. no particular syntax or markdown - unlike Obsidian). It is also not opinionated and fully exportable to other tools.
It is a possible way to "value-add" -- aka be the thing that people can't have unless they pay for it, that strategy is your choice and it's not a crazy or awful one per se. But I will not only always stay away from it, I'll always tell other people to as well.
So far as I have seen, it ALWAYS ends one of two ways: It ends up being open anyway or the company screws people over, eventually. I'm fine waiting for the former and not risking the latter.