You try this with GrapeJS-Editor… I deactivated it and chosen the classic-editor - this one is more robust and without so many bugs.
Does the TrullyPersonal email template have also the mjml version? I suppose not, as the file email.mjml.twig is missing inside the template folder.
If I remember correctly, you can’t use MJML right now for “looking like personal” mails. Those are mostly left aligned and afaik you can’t have a left aligned (not centered) layout with MJML.
It should be possible to add a button as its own element as @techbill mentions.
However, as @dirk_s points out, MJML emails will always be centered. This is a limitation of MJML. So we have to use HTML emails to get a “personal” look.