I understand how demoralising it can be, but I wouldn't recommend making a hasty decision to rule out applying to MC firms based solely on this experience.
Here's why... I graduated in 2019, recently completed the PGDL, and am a career changer. I don't believe MC firms overwhelmingly prefer undergraduate candidates.
I applied to CC last year, passed WG and didn't get invited to VI, however, this year I have. I have applied to FF and got through to the AC stage. However, the first MC firms I applied to were Slaughters and A&O, both of which I was rejected post-app. If you make generalisations about a set of similar firms, based on a PFO from one firm that is in that silo, I would think that you're shooting yourself in the foot.
One caveat, I do seem to get a lot more traction with US firms, I don't know why, but I think from speaking to friends already in the law, these firms do often tend to go for graduates and those who have worked in other professions.
In essence, don't be discouraged. Keep applying to MC firms and even consider CC opportunities. There isn't a one-size-fits-all profile they seek, so persistence can pay off.
Drop me a message if you want