I've tried many combinations over the last 10+ years, but there are many variables involved; things to consider. Here are a few examples.

how many products do you sell?
how often are they updated?
how often is support needed?
does each product require a lot of ongoing maintenance?
is the market saturated? (which it is)

With themes, I've seen single theme purchasing, memberships (access to all), recurring subscriptions, and even a combination to give the customer the option when they visit your website.

However, there is one more that I've seen with a couple of theme shops where you pay a setup fee and then you pay a monthly subscription to maintain updates and support. Not to name any names, one site charges around $35+ as the setup fee, then it recurs monthly at around $8. In a year, that is over $120. I guess the idea behind this is that it gives the customer the choice of when they are confident enough to cancel without the need for updates or support.

There are a small few who offer free themes but charge a fee for support.

Many models for a plugin or theme shop to adopt, but one needs to answer those questions I listed above. Then there is also what the shop owner feels is best for their website.

For Rough Pixels, I've been running with single theme purchases (good for a year), as well, offering a couple of membership options. I do this because there are some people who simply want just 1 theme while others want the choice to easily switch to another theme from the choices they have all rolled into one package.