How important is it that all cables come from the same manufacturer?

I have tried many different combinations. In general, different are absolutely fine. I have been fortunate enough to have components that are perfectly neutral, matched and exactly this is what I want to have come out, so as transparent as possible. So Transparent brand are perfect. a full weave of these is perfect and complimentary. However, even with this. I found a Cardas power cord was better for the amp.

Ideally your components are complementary. Because if you add a first interconnect that attenuates the highs a bit, then the second attenuates the bass a bit. Now as you go through you may be balancing the sound but reducing the detail.

I am 60/40

60% Signal Cable 40% Cardas Audio

Some argue that a Loom (all one brand) is best and you have continuity across your system.'

I say meh.

The better your system the more important it becomes, IMHO.

I am totally mixed and matched here no two cables are the same and everything sounds great.

You can mix brands or different lines within the sane brand.
Tweak the sound to your liking. if you find a cable that made a jaw dropping difference you can try full loom of that brand/line. But…it doesn’t always work. Could become too much of a good thing. All depends on system, room and what you’re looking for.