Hello Alex,
Few months ago, I was also having some similar issues, with respect to GFSK modulation. I was using a M&M TED and I was getting a loss percentage of about 7% mainly due to having a large amount of tx packets (100 pkts/s) . So, it was suggested to me to use a correlator sync block to pre sync the packets, and then do timing recovery.
But, since I also couldn't find a M-fsk or MSK modulation class I start doing some research and I found that the timing recovery block works perfectly if there are no noise in the demodulated output, so I created my own version of demodulator which outputs values only of signal is present, and in case of noise I just output some negative value, say like -5. It just a concept if signal power is above a threshold then it is actual tx if not it is a noise
I am not sure if this can solve your problem. Thought
just in case you may find it useful. Cheers.....!