I am using a fairly inexpensive SSD - and connecting it with a 2.5" drive bay. Very happy with the price/performance:
SAMSUNG 870 QVO SATA III 2.5" SSD 2TB
https://www.amazon.com/gp/product/B07CQ6C4MW?psc=1
The bay has room for 2 drives - at some point I'll be adding a 2nd identical drive - and mirroring my drives
Have you considered using Adobe Bridge? https://www.adobe.com/products/bridge.html
While we may implement a similar functionality in PhotoPrism eventually, it's not our goal to replace Adobe Photoshop & Bridge for what they were designed: Editing photos in high quality & going through a huge list of (RAW) files to select the best shots!
Even if our albums would be optimized for handling 25k+ pictures: PhotoPrism will still needlessly blow up your index for pictures you don't even want to keep. It will create a JPEG for each RAW file, consuming storage.
Indexing is an expensive operation that detects faces, gets location information from GPS coordinates, generates titles, etc.... you should do that later if your intention is to delete most of the files. At the moment, PhotoPrism clearly is the wrong tool for this use case.
This warning applies to most self hosted services, not just photoprism. It doesn't indicate that the app itself is insecure. https://www.cloudflare.com/learning/ssl/why-is-http-not-secure/
A reverse proxy is pretty standard for self hosting. Common options are Nginx, Traffic, Caddy, etc.
We recommend PhotoSync for syncing your mobile phone: https://photoprism.app/partners
It costs a few bucks, but it's the best app we've found. We also use it personally. The iOS version even preserves the favorites flag when you upload to PhotoPrism.
Alternatively, you can use any other WebDAV-compatible app, and of course the regular Web upload.