Cool! That’ll make things easier. The link that I found linked from one of the links you provided linked to some unmaintained repo that said not to use it (by my quick read, anyway).
You didn’t offer a budget and no one else seemed interested. My wild guess on a budget was $2000-10000. Lots of ways to make the budget on the low end by hard-coding things and, maybe referring to products already created on the BTCPay side of things.
A quick solution would be to use WooCommerce and WordPress. It’s a bit fussy, but if the problem you’re trying to solve is “how can I accept BTCPay with Discourse” then WooCommerce would probably be easier (you don’t have to tell me the reasons to avoid WordPress! and WooCommerce isn’t free)
True enough! Whether a decade makes my point any more or less hasty is not an argument I’ll enter unless we’re both armed with beers in our hands 
. And maybe it’s true that Discourse.org would have thousands more customers if only they had a BTCPay plugin, but until paying customers ask them for it they aren’t going to do it.
You might be able to find someone young and hungry for a programming project (perhaps for a course or One Hour: Ignite. Inspire. Code.) willing to take it on as a proof of concept.