You can make Timeshift backup data too by selecting the Include Everything option as mentioned. The developers say that you shouldn’t, because ideally OS backups should be separate from data backups, but you can if you want.
If they’re separate, then it’s easier to restore your OS (eg. if you edit a system config file and can’t boot afterwards) without also reverting any documents and such you’ve been working on since then. Or to restore data from a backup (eg. a file you deleted) without also reverting your OS to that earlier state. Keeping your OS backup (for system restore purposes) separate from your data backup (for retrieving old files) makes that easier.
Still, there’s no reason why you can’t use Timeshift to do both in one. If your goal is to make a full backup of your computer in its current state, data and all, to restore in case your SSD dies (like Clonezilla would), then the Include Everything option will give you that.
You might have an easier time converting to a different format which Proton Pass can import, rather than the Proton Pass export format. eg. Convert your file to the Bitwarden JSON format (Bitwarden provide examples and documentation on their format) and then import that file into Proton Pass instead. (IIRC they support Bitwarden’s format)