Hi Welcome to DUC
You purchase a perpetual license (
https://shop.avid.com/ccrz__ProductD...sku=DYNAP20000) or subscription (
https://shop.avid.com/ccrz__ProductD...ate=DetailView)
These supports the latest versions and goes back several years so should work with versions compatible with High Sierra. Compatibility info for you is here:
https://avid.secure.force.com/pkb/ar...tibility-Chart
Before buying something try the free one month eval, I expect that works with versions you need as well. If not then you can buy a one month subscription to test.
You can leave First installed while you play with standard, but after a while you'll want to remove it, they don't play well together, especially if you add third party plugins to standard (and who would not?).
Either purchase will now include the Avid Complete plugin bundle and Heat (so if you currently subscribe to the plugin bundle for First you should cancel that).
You can also look around for used perpetual licenses, especially if you knew you were going to freeze the system as is and not expect to upgrade for years. I would discourage doing that unless you really know what you are doing with Pro Tools licensing, including risks of being scammed, and a very real risk you may not ever be able to update that licenses.
And as soon as possible plan on moving any Pro Tools First Projects over to Pro Tools Sessions.
And I'd encourage you to get a physical iLok and use that, just likely more reliable than cloud based iLok licensing.
When comparing perpetual and subscription costs make sure you understand what happens if you do not keep paying for support/updates on that perpetual license... the license will keep working up to the then latest version of Pro Tools, but you stop getting license for newer versions, and there is currently no way to re-up or catch up the license to get new updates. Avid just ended a promotion to let folks do just that, who knows if it will ever be back. Avid really wants folks on subscriptions.