SH has more variance than FR. SH is also harder postflop because you get in spots with marginal hands very often but it's generally assumed that higher winrates can be achieved because you can push your edge further than in FR.
However, as I said you do require decent postflop skills more so than in FR. If you want to switch to it you'll probably end up being breakeven or slightly winning/losing for a little while as you learn, but you can also learn by playing FR then switching later when you feel confident about your game. If your plan is to switch to SH eventually imo you should switch to it right away.
As for HU, the variance is probably higher than FR in general, but there's a common misconception that it's insane. The variance is heavily dependant on the opponent you are facing. Ex: Against a very passive player (calling station), it would probably be one of the games with the least variance in no limit, while as vs an aggro reg with 5bet shove dynamics the variance goes through the roof. You require good postflop knowledge same as SH and you have to be very good at finding reads and adapting to the person you are playing against. If you're someone who has a hard time concentrating and not paying attention when multitabling (playing like a robot), HU is appropriate because there's no downtime and it keeps you on your toes.
I'm assuming from the limits you are playing that you can't play NL50 HU (and HU limits below that have near unbeatable rake), so I'd recommend HU sng since at 75bb+ the structure is very similar to cash game.
Edit: Forgot to mention, without a solid background, HU would be pretty hard at first and also it's much more tilt inducing than the other games because you are prone to take beats or mistakes more personally.