Sell it like a normal product. This was the way commercial software worked for decades. They buy a license. They download it and register it while connected to the internet. They then take it offsite. Then, have an option to be paid to be on call for support when a meet is happening. That way nobody is surprised if something doesn't work.
When you say normal product, do you mean users will have to buy the next version? We should be responsible developers with the ability to patch insecurities in the software we sell. You're also going to have feature requests you'd like to publish to your users. As a sole developer one evergreen version seems much easier to maintain for both these concerns. And I think a subscription model or some recurring payment mo…
For OP's kind of software, I'd say one time license purchase would be the most user friendly. So yeah, sell it like people did in the good old days, before the cancer that is SaaS took over the world.