Hello @Pajaelis390, welcome to Sonos Community!
Quite a few things to mention here, I’ll just start by directly answering your bullet points.
I think I can set up the rear speakers using a Sonos AMP that will connect to the ARC, but then how do I connect the third party subwoofer - do I connect it through that the Sonos AMP that the rear speakers are connected to or does it somehow connect to the ARC?
When a Sonos Amp is being used for Surrounds, the 3rd-party subwoofer port is disabled and It is also not possible to set up a 3rd party subwoofer with the Sonos Arc.
I’ve read elsewhere on these blogs that if I use the TV remote to control the sound of the TV, that the ARC volume changes but the rear and subwoofer volumes need to controlled separately on my Sonos App. Is that true? Is there a workaround for that?
This is not true, as part of a home theatre setup the volume adjustments occur on every Sonos product that is part of it.
I am not sure that Sonos is the right choice for you here, considering the devices you already have, but I will share with you the available options if you choose this path:
If you want to purchase a Sonos Arc, you will not be able to use your 3rd party subwoofer, you will need to purchase the Sonos Sub.
If you want to use your 3rd party subwoofer, you will need to purchase a second Amp which you connect directly to the TV via HDMI, instead of purchasing an Arc. You will also need two more passive-speakers as front speakers of the setup, connected to the Sonos Amp along with your 3rd party subwoofer.
To avoid confusion I am showing below the two possible setups and with bold the things you would need to purchase:
Sonos Arc + Sonos Sub + Sonos Amp (for the rear speakers)
Sonos Amp (connected to TV) + 3rd-party subwoofer + two front passive speakers + Sonos Amp (for the rear speakers)
For the subwoofer, if the setup in #1 doesn’t work for the TV, would this set up work for music at least?
As mentioned above, this is not possible with a Sonos Arc, but it will work with the 2x Sonos Amp setup.
I hope this helps.