I’m eligible for the US Lifeline program to get a phone number for free. I would use it on websites that require a phone number, like Discord, which doesn’t accept numbers on services like Google Voice, and replace my personal phone number with it wherever necessary. And I would start using my personal phone number only with friends and family. I would NOT be using a provided lifeline phone, but rather a sim on my iPhone.
In the US (OP’s post was somewhat US-specific), I haven’t had trouble paying cash for prepaid sim cards. Those services tend to be on the expensive side, but I suppose you don’t have to keep the number active after your Discord signup.