Caution! A TWRP backup in general doesn’t backup personal data of the user such as pictures, ringtones etc., whether that includes contacts depends on where exactly they are stored in Android, I lack a deeper understanding of that yet. It is safer to assume TWRP doesn’t back them up.
Here’s a post with a screenshot with TWRP explicitly giving this warning while backing up …
I sync my contacts locally with Thunderbird using MyPhoneExplorer (Windows only). But I wouldn’t need to sync them, MyPhoneExplorer could just store a local backup of them from the phone. As far as I could see until now, all data fields I have in the contacts are preserved.
Besides, I use MyPhoneExplorer to backup the complete Internal Storage part of Android which TWRP doesn’t touch.
Together with a TWRP backup of the OS that should cover most things, an open question to me were App settings, I would have to revisit that when I need to restore the phone completely, I don’t want to try that just for fun now 
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