Does this pass ACX standards?

[Trebor]

… I do noise reduction on some tracks if needed where the noise floor is -55db or so …

My suggestion would be use noise-floor of -70dB, rather than the bare-minimum -60dB specified by ACX.
That would probably mean employing some Noise Reduction on all your output, (the least necessary to get you close to -70dB).

For expansion - this is a new concept and I do not understand it but am willing to learn about it.

It’s similar to a noise-gate. It pushes down the noise-floor, but only acts when you’re not speaking.
Must avoid setting an expander/gate to squelch the noise-floor to minus-infinity, as ACX don’t like that.
I believe you can get away with using an expander to attenuate the noise between words by 6dB without listeners & ACX noticing you’ve used processing to do that.

So with these two tools - will it eliminate the ‘hissing’ noise you mentioned earlier that would hold me back from a professional sound?

With Noise reduction & expander effects IMO your noise floor can be -70dB without the audio sounding processed, (i.e. the effects have been applied transparently).

Also how did you do the notch-fill in?

I used Audacity’s filter curve to push out the notches ,
(but there is a free AutoEQ plugin which will automagically remove such dents).

And what exactly does that do?

Removing the notches in the spectrum removes your room from the recording.
(the physics of the room added the notches via destructive interference).