I ended up solving my own issue.
I gave the middleware approach a shot but it didn’t lead to any dersiable outcome, all the caching headers I put there were being ignored and I just ended up with 100,000s of middleware calls.
The solution in the end was very simple, treat my website as a Static Export.
Removing the middleware, removing ‘force-dynamic’, and adding output: 'export' to my next.config.js worked perfectly fine and dropped my usage significantly and obviously proved that my pages were not ISR as that’s called out as not-supported for Static Export.
So all in all I still don’t know WHY vercel considered my pages as ISR, but at least now I fixed the usage problem.