Does the bias amplifier of the RLD amplifier need to be connected to reference electrode of the measurement sensor?
Your MyoWare AT-04-001 muscle sensor may already have a connection for a reference electrode:

Page 5 of the datasheet you linked to shows how to connect external electrodes to the AT-04-001.
That sensor has all of the electronics needed to amplify the signal and to do &quot;right leg drive&quot; (RLD) as well as to do some preprocessing of the signal.
The output signal is amplified such that the output voltage ranges from 0V to the supply voltage of the module.
Adding amplification to that with the ADS1299 seems like overkill.

I have no experience with this kind of stuff, but from the datasheet it seems that the ADS1299 can completely replace the MyoWare AT-04-001 if you add electrode connectors.  You'll need a PCB but then if you are using the AD1299 at all you will probably already have had to make a board of some kind.

I would suggest testing the MyoWare AT-04-001 with external electrodes and a reference electrode.  I would almost bet that gives you a large improvement right there.
If that doesn't work, then consider using just the AD1299 with three electrodes (middle, end, and reference as described in the Myoware datasheet.)  The AD2199 should have all the gain (amplification) and other things (RLD) you need to build your own EMG.

The right leg drive cannot, of course, be used with the FSR400.  The FSR400 is just a variable resistor and cannot make use of RLD.
If you use RLD with the ADS1299, then you can't use the FSR400 on the same ADS1299 - as I understand it, the RLD applies to all channels, which would have the bias for the EMG messing with the FSR400.  You'll have to figure that out for yourself.