I don't think PRQ is going to care what you post on the site. They take a hands-off approach to web hosting, and I commend them for it.
But making the site unusable in the sense of posting any and all images (cats, dogs, cartoons, etc.) to make the site unusable sounds like a decent option. Just make the signal:noise ratio as bad as possible.
To explain real quick, 95% of calls don't pick up. Phonebank Buddy let's you skip to the next call in a single click, as opposed to the old and slow way using the CallHub interface.
I put together this documentation to help get even the least tech savvy volunteer up and running in 3 steps.
https://www.notion.so/Phonebank-Buddy-d0318736ca6243aa8df6601c528259f8
Slack, Discord, and FB Groups are all terrible ideas. They offer zero security.
Look into something like this. As far as I know this would be your best bet.
Absolutely.
Your isp will be able to see you're using tor and thats it. You may be able to get a vpn such as NordVPN, which is based in Panama, to mask your IP then use tor on top of that. With this route only Nord knows you're using tor, and they have no obligation to provide that info to your government. Double check what Im saying but you should be good then. I know tor is illegal on some countries.
I cant speak for what the U.S. is capable of, but I think the biggest vulnerability to you is your clearnet traffic and this discussion. The only guaranteed safe thing to do is do nothing