Which OS are you running on the Pi4? If it supports an Allo DigiOne Signature HAT ($250), as mine does, you could use it as a Roon endpoint (in addition to rooEXtend) with great sounding SPDIF out into your M gear. Also supports Airplay and LMS along with Roon, so that she can just stream from her phone or tablet.

EDIT: Actually, the original DigiOne ($110 from Chicago Electronics) would be fine for her AirPlay use.

Server: Audiolinux Roon NUC w/HAF room correction and crosstalk cancellation
Theater: 218, Allo Signature, miniDSP, HD621, C61R, DSP5200SEu, 420, 33, NHT subs
EtherREGEN w/Finisar fiber, Shunyata Denali
Living Room: RPi4 Diretta Host & Target, Ghent USB > Singxer SU-6 > AES NA M-Links > DSP9
Shunyata TritonV3, LHY lps, Farad Super3, Stack AUVA feet