First, don’t stress over it. Most instances are not strict on only federating with guaranteed instances. Most do not auto-sync with Fediseer at all, and the ones that do are more likely to only be syncing censures (when other instances are reporting the instance as problematic).

To get guaranteed on Fediseer, you need another instance to guarantee you. If you start your instance, hang out in the spam defense chat, and are generally sensible with your instance, then you’ll find someone willing to do it no problem. Guarantees are not a huge risk to an instance since they can also be revoked at any time. If someone guarantees you then you start being a dick, they can just remove your guarantee. So it’s not a big decision, people wil be happy to guarantee someone who seems reasonable.