wouldn't it basically need to be the same as what they use for intercontinental cables ? tons of shielding and repeaters. maybe there'd be further risk bc if it breaks at the ship it could fall on or weigh down the submersible

Not at all. Shielding against what? It's light. You only need enough of a protective coat to make sure it's stiff enough to not twist and stretch too much and to make it neutrally buoyant. No repeaters needed, base range is about 40-60 km. ROVs going to greater depths use them all the time without major issues.

The real reason is that they were stingy as fuck and that it's mildly impractical which outweighed their complete disregard for safety.

> intercontinental cables

Those also carry electrical power to supply the inline repeaters on those cables. If the cable does not have repeaters, it wouldn't need this, and the shielding could be greatly reduced.

Would be extremely heavy and very brittle

There are deep sea submersibles designed to go a good bit deeper than Titanic which have optical tethers.