So in this situation, I'd first make them private, because if you delete them completely you won't be able to get any data from them. First, because they're that old, first go to the video analytics and get the ctr. Sometimes just because you think the thumbnail isn't very good, doesn't mean it won't perform well. Look at the ct and note down what they are.
after that, take a screenshot of the audience retention data. I'd make sure you audience retention data is set to "lifetime" you don't have many views on those videos, but that doesn't mean they don't have ANY data, so take some screenshots of that for future references. If the audience retention looks like hockey sticks, then you're not missing out on deleting them anyway.
BEFORE DELETING. Download the video because you'll probably have lost where the original video has gone.
Re-edit the videos with copyright free music from the YouTube Audio Library (easy to find via creator studio)
Then delete the videos and re-upload them with new copyright free music. My rational behind this is you might not have gotten many views for these videos over the last 30-90 days, so it's likely new audiences had never watched those videos, meaning you can re-craft them into new content.
overall you won't be losing many views, and you may even end up getting new views from re-crafting them with new copyright free music.
I'm planning on doing something similar with content I never actually published and they've just been sitting around since 2016.
And on one final note, it's nice seeing an artist from 2016, and it sounds like you're into metal too, which is always cool in my opinion.