That guidance about running PsychoPy files on other servers only applied when the feature was first being demonstrated as a limited proof-of-concept. It has since matured and a robust online service is now available. So if you want a free alternative to running PsychoPy online, you’ll need to find an alternative to PsychoPy itself.
If you already know PsychoPy, then the question becomes whether 20 p per subject is somehow worth more than the time and effort you will put into finding an alternative and then learning to use it… I think you might be undervaluing your own time and expertise substantially.
The individual credit price was set so low that it would not be a barrier to most users but in aggregate would provide an income stream to employ developers who would be able to greatly improve both PsychoPy and Pavlovia. The site licence fee, however, was never intended to be paid for by individual users: at £1500, it only becomes cost effective if you are running more than 7500 subjects per year. It is usually paid for by a university, and then everybody within that university is entitled to unlimited use within that year. So you could also explore whether a department or research group within your university is willing to purchase a site licence, which you would then get free use of, simply by registering your account with an e-mail address corresponding to that institution.