Is there a way to block email tracking services on Apple's Mail app OTHER than blocking all remote images? Any extensions?
I don't have an answer to your question. However I do have information that relates to this issue. 

Even though you might have images turned off on your mail, apparently Spotlight follows the urls during the indexing process, which negates the advantages of having the images turned off. 

So, you might want to turn off spotlight indexing of your mail. :( 

Link below.

https://lifehacker.com/disable-email-search-in-spotlight-to-avoid-message-trac-1678528323