So I think you either somehow limit the relative value of such companies

If things like anti-trust, monopoly, anti-consumer laws were properly enforced that would at least help. Meta shouldn't have been able to buy Instagram or Whatsapp for example. The Paramount + Warner Brothers merger should never happen. Its not an entire solution to the problem, but it's a start, and there are already existing laws to make it work, but the regulators (edit and courts) have all been captured (which itself is a side effect of capitalism)

There could also be something about big companies getting split into smaller ones in ways that make sense as a company gets too large. Like maybe Starlink is eventually required to be spun out from SpaceX even though that was done internally. You could maybe even keep some of that 'founder' mindset by having someone internal that worked on the project from the start take it over in the split. Maybe the original founder could still have some input as well, but you'd make them divest to a certain level so it's not just 2 companies, but still being owned by the same person.