Here is what I'm doing that works
RewriteRule ^sitemap-a\.xml$ https://www.physicsforums.com/sitemap-10.xml [NC,L,P]
RewriteRule ^sitemap-b\.xml$ https://www.physicsforums.com/sitemap-11.xml [NC,L,P]
RewriteRule ^sitemap-c\.xml$ https://www.physicsforums.com/sitemap-12.xml [NC,L,P]...
Thread prefix still isn't working. I put in the prefix id, fill out the rest and hit create thread and it refreshes with "Please select a prefix." in yellow. I added the the code to functions.php. Happens before I publish and after.
Just returning to this topic. I calculated the current source code size in bytes for a thread with 100 posts, which is approximately 1.7 MB. If I remove whitespace, the size reduces to around 900 KB. Makes quite a big difference.