it is subscription, you pay per user per month. It is quite cheap imo, but I understand if you don't want to pay any subscription. But in that case you can just try to self host it instead.
I've used odoo before, it is a large piece of software and can be modified to do lots of things. Most likely you will be able to get it to do what you want. You'll probably need the e-commerce module, there is probably some sort of mode for subscriptions. You can also add the CRM on top for marketing, etc. there is also the booking module (iirc) which is maybe useful for sessions with trainers etc. maintenance might also be useful.
Using windows is also massively wasteful, it spends probably more than half of the processor usage on analytics and tracking stuff. Linux or other free kernels are much better in terms of power usage, on old and new hardware
tbh I'm scared of the power the gov already has. I am worried if I join any communist org, it will endanger my partners immigration status. I also don't see the orgs around me doing much useful stuff it seems, mostly just selling newspapers (that no one buys cos they cost a fiver)
See if an office equipment seller is doing office clearances and you can get some office chairs for less than $100 most likely (I got one for £50)