Most Intel GPUs are great at transcoding. Reliable, widely supported and quite a bit of transcoding power for very little electrical power.
I think the main thing I would check is what formats are supported. If the other GPU can support newer formats like AV1 it may be worth it (if you want to store your videos in these more efficient formats or you have clients who can consume these formats and will appreciate the reduced bandwidth).
But overall I would say if you aren’t having any problems no need to bother. The onboard graphics are simple and efficient.
I switched to Immich recently and am very happy.
The bad:
Honestly a lot of stuff in PhotoPrism feels like one developer has a weird workflow and they optimized it for that. Most of them are counter to what I actually want to do (like automatic title and description generation, or the review stuff, or auto quality rating). Immich is very clearly inspired by Google Photos and takes a lot of things directly from it, but that matches my use case way better. (I was pretty happy with Google Photos until they started refusing to give access to the originals.)