I put this together based on another very similar script (attributed, of course) I found on beehaw that forces external links to open in a new tab. Should work on every Lemmy instance by checking the header. While it uses http*://*/* as the match, it only executes any logic if the isLemmy check returns true.
Honestly, this has improved my desktop experience significantly, it was killing me before.
Hope it helps, feedback/suggestions/contributions welcome!
Gonna try this out when I get home. Thank you, this has been bothering me.