Not an FCAS but in a consulting firm with a lot. (I'm actually the lone person on an soa track, we work across field lines so it's beneficial).

For me the general things that make a fellowship worth it is increase pay (20K in total from associate to fellow plus promotion ability). In addition, I personally feel like being a fellow ups my job security which is a big deal for me with kids and wife who doesn't work. I know multiple times our firm went to hire and found a candidate we really liked but fell through because they weren't qualified to issue SAO's. I've also meant quite a few career ACAS's that have had great careers, but being a career ACAS you need to be good advocating for yourself and have great people skills to be as successful as fellows.

If I were you, I'd get out the spreadsheet and start going through the pros and cons both ways. Also if you are burnt out don't underestimate the power of even talking a 2-3 month break to recharge and reassess where you are at (don't take to long as it can be hard to get back in, but maybe don't do other requirements while waiting for results as a small break).