The list unsubscribe header is there, you don't need to do anything.
Only when you send a test from campaign --> template --> test template the unsubscribe heder is missing.
So if you send a test email from inside the template to mail-tester.com it will always say the list unsubscribe header is missing.
If your messages from mailwizz go into spam there's a different reason for it.
Roundcube = personal email
Mailwizz = bulk email
Give it a little time, new IPs and new sending domains can get you into spam folder. some decent opens on your emails and you may see improvement. Warm up your IPs, start sending slowly and try to get your subscribers to interanct. Opens and replies to emails are a good signal to ESPs.