I'd like to share with you a project that I've recently published on Github and might interest WISPs in our community - a captive portal written in PHP/SQL named WIFIS. A very lightweight system can be installed on a Raspberry PI (2/3/4) and supports user registration, subscription (Paypal integration) and user statistics. It is free to use (MIT license) and build upon.
The system was made in 2019 as an experiment for a local WISP network. It does not have any administration build in (for now), so you have to manage it via RouterOS and SQL. It works on desktop and mobile and is multi-lingual (English, Italian, Slovenian and Croatian).