The thing you actually want -- keep every text message, drop attachments older than N days -- does not exist in Signal as a setting. Here's what does:
- "Keep messages" / "conversation length limit": deletes old texts too. Useless for this.
- Settings > Data and storage > Manage storage > Review storage: sort by size or date and delete by hand. Free, but deleting an attachment deletes its whole message (captions included), and you're tapping through 16GB.
- "Optimize on-device storage": exactly the right feature (old media becomes thumbnails, texts untouched, re-download on tap) -- but only if you pay $1.99/mo for Secure Backups.
- Desktop: no storage UI at all.
The paid part I get: hosting 100GB of media costs Signal real money. But deleting files off my own phone costs them nothing, and that's the part that's paywalled. A free "delete media older than 30/90/365 days, keep messages" toggle would fix this for most people.
Least-bad free move tonight: on-device backup first, then Review storage sorted by size and kill the top few dozen videos. That's usually most of the gigabytes.