how to get round the crazy 5000 shared album limit with my 2TB plan?

Had you considered having a separate shared album for each year? You are allowed to have as many as 200 of them.

Shared albums don't count against your iCloud quota in the first place, so your 2TB plan is irrelevant here.

If you really want to have copies of a photo in both your private library and your shared library, you can do that, but Apple doesn't make it easy. You have to duplicate the photo, then move one of the copies to the shared library. Here's where your iCloud quota will be important, because you will need room to store both copies.

Have you tried creating another shared album and adding pictures after you have 5000 total shared photos? My reading of Apple's document Shared Album limits - Apple Support indicates that the limit is per album. If you saw an overall 5000 photo limit somewhere, where did you find it? If you have tried it already and failed, I bow to your experience.

I don't work for Apple. I didn't make Apple's rules. I'm just one of the group of users here trying to help each other interpret how things work and how to work with them.

There are several other limitations of the legacy Shared Album (originally Shared Stream) system that others find make it unsuitable, including limited resolution, filtered metadata, limited format support, and very limited sorting.

If you want Apple to consider your recommendation about extending shared albums and making the subject to storage quotas, you should make them through their official portal for such comments: Product Feedback - Apple. I'm not sure whether that should go in the Photos category or the iCloud category.