I have a new(ish) Giant Roam 1. The whole bike is Deore but has MT200 brakes which I've read is bottom of the barrel. Most of what I do with it (which is why I bought it) was for rough Country roads, and rails to trails and such so the brakes have been good enough so far. Today I did a ride that had some more challenging descents on some gravely hills and I guess the brakes worked fine but didn't feel like I was confident in their control, they felt plastic and flexy if that makes any sense.

I fully understand that it just might be that I need to learn to control them better but I was looking on line at parts to upgrade. A lot of the newer/higher end models are quite pricy but there's some older Deore M505 and M596 that are a great price point.

I guess my question is are the older Deore designs better than the new MT200 since I know Shimano trickles down technologies, so is it a worthy upgrade?

IMO M596 is the nicest as it is derived from higher end gear while MT200 is evolved old gear; M505 is a derivative of failed ergonomics known as MTB brifters. The 1.5 lever ergonomics are the best. There is enough power for 1 finger braking, the stiff and stubby lever does not flex, and Servo-wave gives better modulation. I-spec is great for eliminating handlebar clutter and offers great ergonomics.

I have a set of these M596, as well as SLX and XT sets. I really like the Deores, they are not as powerful as the XTs, but I like that on my hybrids tire's smaller contact patch. The difference in the lever is tool-less bite adjust and XT calipers are different, with nicer pads.

If you have flat mount, you might want to keep your calipers and just swap out the levers.

Another way to boost brake power is to up your front rotor size (140mm-->160mm-->180mm). If this is a new bike, braking power does increase after bedding in.

An unconventional way to boost power: Change to BH90 hoses; Shimano use a smaller bore hose on lower end brakes to lower the power on certain units.

Have you tried new pads?

I think I've read elsewhere that all Shimano flat bar hydraulic brake levers are compatible with each other. Is this correct?

I think BH59 is the basic hose, right? BH90 is fully compatible, if you change the whole thing out? I guess one would need olives and nipples (and possibly nuts) for BH90 as well (a "full" conversion)?

Yes, drop bar levers as well.

Same outer diameter, different barb due to different inner diameter, same olive and nut. One barb is yellow and the other is silver. Correction: BH90 uses a thinner bore for a stiffer brake feel, BH59 uses a thinner wall (bigger bore) for a spongier feel. They are interchangeable; just use the proper hardware.