I know Credit Mutuel can provide you a physical card with a bunch of codes of which one is selected at random at auth time
Had family use it because they had a Huawei phone

Signal on both Android and IOS
Unfortunately not usable with Linux without a phone yet but so far the most accessible option

Ubuntu Touch has a planned release for September 24th! Eager to see what devices have a full compatibility rating to know what I’ll buy next

I’m in a similar position. My next phone will be a Linux device I just hope I can install Signal everything else I can do on my pc instead even if it’s less convenient

ExFat doesn’t support file permissions so a default value is used, I believe you can only change the permissions at mount time