I am not sure about mixing and matching plans but the AT&T Prepaid service is pretty good. I have had it since 2016 when I switched from my Postpaid AT&T account to the Prepaid account due to somebody hacking into my account and trying to order a top of the line iPhone and then a android phone and have them delivered to an empty house 2 blocks away. AT&T cancelled the phones but could not block the upgrade choice(I buy my phones outright so I don't need an upgrade) so i canned my AT&T postpaid account.
I have my prepaid account set up like a postpaid account where AT&T charges my credit card each month on a specific day rather then me having to remember to reload my account each month.
I originally had a $48 plan with 6GB data and unlimited talk and text that AT&T gave me a discount for using auto-pay so that it came out to be $43.10 per month. I discovered that I waste most of that data(there is a one month roll over) each month so I dropped down to a $30(plus tax and fees) 5GB data plus unlimited talk and text plan when I bought my iPhone 7 (I foolishly did not port my old number over and got a new number that over the last few years has suffered from a crap ton of scam calls so I also took the opportunity to get a new number)
The nice thing about the prepaid plan at the time I got it was that unlike the postpaid plan i had, if I went over the data amount, I did not get hit with a fee, they would just throttle back the speed of the data. On my postpaid account, if I went over the data amount, I got hit with extra charges. I am not sure if they do that still.