The issue that I have with using the more block is that it creates inconsistency on your site and it can look really bad depending on your site design to have entries with different excerpt lengths. I think auto generated excerpts are best to ensure all of your entries match. This is especially important if your theme uses a grid based display for your entries so they display side by side.
Now, in terms of SEO though I think using the custom excerpt field is ideal because you can give your posts unique excerpts and prevent possible duplicate content issues on the site. What I’ve done in the past was simply copy my Yoast SEO meta description into my excerp