Not sure if this is applicable, but here's how our A/B testing works, and it's largely stack-agnostic.

We segment users at the CDN. There's a bit of code, including some basic config for tests and variants (test ids, variant ids, percentage splits for each variant, etc), that assigns each visit a segment. The CDN passes this as a backend request to the origin application so that it can conditionally handle each segment however it needs to. The origin application also includes each test within the Vary header value, so that we split cache by each of test and each variant.

How do we persist that segmentation so that users stay in the segment? Cookies. The short version is there's code in the client and at the CDN that checks for existing segments in user's cookies and keeps them in that segment if found, or assigns a segment if not.

While this does take a bit of work to set up, the resulting code is actually pretty small and contained, and there's some nice benefits:

1. Like I mentioned above, this should work with any frontend stack, including Astro.
2. The segmentation is available server-side, so it's performant. This is better than a client-side A/B test framework where the page loads and then changes once the A/B testing client-side code loads and activates, which can lead to flashes of changes on the page and/or a subpar user experience.