How to backup and decrypt Signal for iPhone message history

I use a combination of Signal, WhatsApp and Messenger for communicating with friends and family. Recently I made a series of transitions from Android to iOS and then back to Android, and beside the annoyance of entering account and banking details anew, migrating messages was the main obstacle in the way of a smooth transition.

I am not sentimental about text messages but I did have photos which I had received from other people and wanted to keep. The only way to do this in the iOS -> Android case was to go through all of my threads in WhatsApp and Signal, save the photos I wanted to keep to my Camera Roll (??) and then copy them to my computer.

On Android, WhatsApp stores its data in the data partition, and Signal can either save photos to its own folder or export a backup, although I did not look into how to decrypt the backup.

Ultimately what drove me back to Android was that iOS did not give me control of my data in the way I am used to, despite some of the much touted privacy advantages.