Well, first off Happy Birthday! I also have a sitdown longarm - i see that you have checked most of the usual list of “offending” culprits! It can be frustrating i know. Rethreading the machine and bobbin would be first and checking for lint in the bobbin area while there. You’ve checked the needle and maybe gone to one with a larger “hole”? I remember using some shinier thread for the first time (trilobal polyester) and got shredding. It was wound on the spool in the same manner as cones are so i used vertically instead of putting it on the horizontal spool holder. After getting a little frustrated and taking a break i remembered i had a thread net - that did the trick! I keep the nets where i always know where they are now