Optiboot has been re-written to work on the tiny-0 and tiny-1 chips, and SpenceKonde's megaTinyCore supports the chip both with and without optiboot.
Once you have loaded optiboot onto an ATtiny3217 (which requires a UPDI programmer; similar but slightly different than needing an ISP programmer to put the bootloader on an ATmega328p chip), you can upload and do application-level Serial communications over the same UART port, just like an Uno or nano.
PROBABLY you need to set up a way to power-cycle the chip, because it doesn't have a "reset" pin by default.
I THINK the source of confusion here is that UPDI programming also uses a UART (with a bit of special wiring); so to burn the bootloader onto a UPDI chip, you connect up some RX/TX thing to the UPDI pin, but after the bootloader has been burnt, you'd connect to the RX/TX pin of the 3217 instead of the UPDI pin.
I don't know of any existing boards for sale, or theoretical designs, that implement an ATtiny3217 + USB/Serial chip to yield an Uno-like board. For one thing, even though the t3217 is theoretically cheaper than an ATmega328p, it would be nearly impossible to produce something that competed with the price point of the Chinese Nano derivatives (or even the official Arduino Nano Every.)