So are you saying you now have everything but a center channel? That's the way your post reads.
Most all audio for video (TV and movies) is now mixed for surround and the center channel is a primary source. Other posts here have given some good information. Music only recordings that have been mixed in surround formats exist and are becoming more popular. Downmixing surround to stereo is not great, it would be better to upmix stereo to surround (if done well).
I worked for over 30 years as a recording engineer focusing on classical and big band jazz. Recording in stereo was a challenge and it never gave satisfying results. The width of the stage and the ambience (reverb) of the halls was never well portrayed in stereo playback. When surround started, I experimented with multichannel recording and playback using 5 channels, L/C/R and two rear surround channels. This was for my own experience, I was not using surround algorithms, just discrete channels for recording and playback. 5 microphones, five channels of playback. I was finally able to more realistically recreate the experience of sitting in a concert hall and hearing the orchestra as it sounded in the hall. Instruments on the left side of the stage were in the left speaker, center of the stage was in the center speaker and the right side was in the right speaker. No phantom image, discrete recording and playback. I had two microphones hung back in the hall to capture the left and right sides to reproduce the ambient sound of the hall reverb and the audience.
Once you experience well recorded surround sound, you will realize how much is lacking in stereo. The history of recording is very interesting. There was a time when three channel recording was used in studios, three track recorders and 3 playback channels (L/C/R). Unfortunately, vinyl records were the playback medium and we went from mono to stereo and three track recordings were mixed to stereo.
My advice as a professional audio engineer is to get your surround system completed, 5.2 at the bare minimum. Two subs are better than one. 5 main channels is the minimum requirement. Start listening to movies and surround music as it was intended and you may never go back to stereo.
Lots of information here:
https://www.routledgetextbooks.com/textbooks/9781138921368/