I know of nothing that should have changed to cause this for 2.5.12. But a work around could be to download the add-ons kar file and place that into the /usr/share/openhab/addons. Then OH will install the add-ons from that file instead of trying to downloading them from the internet.
You can find the kar file for 2.5.12 here.
Since 2.5 there have been a ton of improvements in OH and the rules which should reduce the complexity and number of lines of code for the rules by at least half. For example, conversion between degrees C and degrees F happens automatically so there’s no need to maintain two different sets of rules. JS Scripting now supports a modern version of JS and the built in helper library makes the interactions with OH much simpler.
But I’m not sure OH 4 or OH 5 will ever run anything close to reasonably on an RPi 0 W or RPi 0 W2. The amount of RAM required for OH just keeps on increasing. The minimum supported hardware for OH is now an Rpi 4 with at least 2GB RAM. OH 5 requires 64-bit which makes it even more RAM hungry. Have you looked into RPi compute modules as an option?