I have a DIY harmonic drive 4th axis and haven't found it to be the limiting factor in overall rigidity of my admittedly small-ish machine & setups. I don't have a brake on mine, but I'm using a NEMA34 stepper which has very good holding torque.
Having said that...
I've read - no first hand knowledge - that Haas and some other MFGR's have generally chosen other reduction mechanisms because the harmonic drive can have torsional flex in the flex spline. True, they have effectively zero backlash, but the flex spline is not as stiff as a cycloidal or double-worm drive. I suppose that the flex can contribute to chatter or broken tools under extremly high loads - imagine drilling with a spade drill way off center.
And unlike worm drives, harmonic drives can be back-driven, especially the lower ratio versions. I don't know if cycloidals can be back-driven.
This is why I chose to use a drive size which is significantly larger than is required for my size 4th axis. I probably could have used a 17mm size, but I went with a 32mm - it's about 10x stronger than I need to whip my steel trunnion table around. But it's also stiffer, torsionally, than the smaller drives.
The one reason I can think of where a brake would be seriously beneficial is when a servo is used to drive the harmonic (or other) gearbox. It's possible for servos to 'dither' or hunt in between positions, or if the encoder position tolerance is very high, then the drive could be back-driven a little bit by cutting forces before the servo tries to recover the position. Here's a case where an axis brake would put a positive stop on everything. In contrast, a servo-mounted brake would stop dithering/hunting and back-driving, but wouldn't help with the flex spine being twisted.
In my situation I will eventually add a brake - but to the tailstock. I want the ability to add some drag or be fully locked - because even though the harmonic drive isn't moving or flexing, the trunnion table is still free to flex and rotate around the tailstock.
If you're using a big stepper then those two harmonic drives you've selected will be massive overkill from a pure strength standpoint. If you're using a servo, then only experience will dictate if your particular servos, gear ratios, and system tuning will hold the axis in position well enough without a brake for your setups. Just try to keep heavy cutting forces as close to the center of rotation as possible - the B-platter should be lower than your A-rotation center so the expected part center is in line with A once fixtured.