Steam version and source compromise

I'd like to get Substance Painter for occasional use, and I've been researching the options for purchase.

I think, and please correct me if I'm wrong that I have two options:

 - I buy a subscription. That costs $20 a month and I have to buy other things I don't think I would ever be likely use.

 - I buy it on Steam which is a one off payment. I won't get upgrades (after the end of the year), but I'm only paying for what I need. After a few years, if I want some new features I could buy (for full price) a newer version.

The Steam version sounds perfect but for one thing. As I understand it, I wouldn't get any access at all to Substance Source. I can't even buy things individually (again if this is not correct, please let me know). So my question is this...

Could I buy the steam version, then from time to time, pay $20 for the month to get some assets, and use them in the Steam version forever?

Thanks, and sorry if I'm posting in the wrong place.