FWIW I'm going trough a similar issue, and bought this cheap USB to s/pdif converter which was well reviewed by a member here.
The Phiree U2SX is a simple, efficient and compact USB to SPDIF and Optical to USB interface. It is based on a Bravo SA9023 chip, supporting sampling rates up to 24bit 96kHz.
www.audiophonics.fr
Hopefully it will break the ground loop, which I assume is caused by my laptop's power supply (loop disappears when running on laptop battery).
I'm not sure an audio interface always resolves ground loop issues BTW, because I briefly had an audio interface (Motu) before returning it ; at the time I hadn't identified yet the intermittent SQ issue I perceived as ground loop-related, but in retrospect I'm pretty sure it was there, as I swapped between interface and unbalanced DAC, with either device, to decide which I preferred (both sounded... identical).
There are many "hum destroyer" boxes on the pro audio market, RCA such boxes are less common. A member mentioned a Jensen box. If you're in Europe, I found this
https://www.oehlbach.com/en/audio-equip/electronics/audio-linear-8. I didn't buy it, going to try the Toslink solution.