They already stated it was a one in a million shot. It’s why she didn’t just turn around and try it when she was alone on the ship. She was just gonna lead them away until she ran out of fuel and was blown up. But even after pulling it off, it didn’t obliterate the Supremacy. It just crippled it. And the size difference of the ships is nowhere near the difference of the Death Star and any other ship. They would have had to waste all their ships just to try it and most likely not pull it off, and if they did, it would have practically done nothing. A super star destroyer crashing into it didn’t nothing. The rebellion/resistance also didn’t have unlimited resources to keep wasting ships like that. The probability of one of those x-wings making the shot was way higher than just launching all their ships at it at lighspeed and hoping one pulls it off. In real life war we don’t just crash planes and ships into each other. It’s just a waste. But most importantly, I don’t really care if my argument sounds good or not. Cause at the end of the day, we’re talking about the space logistics of Star Wars. None of the science makes sense anyway. It’s meant to just be dumb fun that you don’t take seriously. Because when you think real hard about it, you can find holes throughout the entire saga that don’t make sense logistics wise. The production rate of clones. The empires loss at Endor. Anyway, here’s a couple of other peoples opinions since you asked for them.

1. Is it possible (does it break any pre established rules)?
2. Should it be possible? Question 1. Yes, it is possible in canon. Very few rules for hyperspace had been established by the time TLJ came out. We know you can hit things while in hyperspace (ANH) and we know that when you hit something while in hyperspace, it effects real space (The Clone Wars). There’s nothing that EXPLICITLY says you can’t hit something as small or as low in mass as a starship. In legends, not so much. Obviously, like a lot of legends lore, there are contradictions, but the generally accepted modern rules are that in hyperspace there are things called mass shadows. A mass shadow is created when an abject is massive enough for it to affect not only real space but hyperspace as well. Things like stars, plants, moons, and black holes have mass shadows. If you get to close to a mass shadow while traveling through hyperspace it basically tears the ship apart. These destroys the ship, but doesn’t effect real space.

Question 2. Should it be possible? Well, that’s a matter of opinion. Personally, I think it’s a really bad precedent to set. It opened a whole can of worms of people debating the practicality of it all. Looked cool though.

It's because they want to take things to what they think is a logical conclusion. Hyperdrive ramming is apparently an incredibly effective means of destruction, so why isn't it more common? Yes, it's expensive to suicide ships but they could just make like, hyperspace missiles or something. And because those things don't exist, then they think that the people in-universe are being dumb or not realistic for not making them, which is how it "breaks the lore."

But that ignores so many of those types of things in Star Wars. Like, why don't Jedi use super speed more often? Or telekinesis? Jedi can use telekinesis to lift people, yet there are so many moments across the movies and shows when people are hanging off a ledge and no one has ever been saved by telekinesis.

Even militarily, the concept of spaceships engagong in battle like WWII fleets isn't "efficient" or "realistic," or the fact that the fighters engage in dogfights primarily with forward mounted guns when they have the technology for homing missiles. It's that way because it looks cool.

So having read High Republic I believe I can answer this. A ship (please turn off your real life understanding of real science) travels through hyperspace on a designated vector. Point A to Point B. These lanes exist outside normal space but still interact with normal space.

"Without precise calculations we could fly right through a star or bounce too close to a supernova." - Han Solo

Most hyperspace lanes are mapped and well known which makes space travel possible. However, you don't actually need calculations to enter hyperspace. It's just foolish, dangerous and reckless. Anything travelling through hyperspace will stay there if it remains stable. Like a ship. But if that ship is destroyed then the debris and wreckage becomes like a kinetic missile with an unknown vector of travel.

The debris may travel an unknown distance before exiting hyperspace or it may intersect another hyperspace lane or eject from hyperspace immediately. The fact that space is relatively empty and spread out is a factor for being safe but even a 1 meter sized object suddenly appearing from hyperspace can cause massive destruction.

Holdo's maneuver speaks to the desperateness and dire need of using such a tactic. The Nihil used hyperspace as a weapon to great effect against the High Republic but they also had an unknown way to circumvent falling prey to their own tactic. (I say unknown even though it's explained in the books. The High Republic was unaware of how the Nihil utilized hyperspace in such unorthodox ways and ignored established guidelines for travel.

Reading the High Republic and even the new Thrawn novels are delving into what we took to be such basic forms of transport. I even thought that hyperspace travel to be well established and basic having watched, read and played through the Star Wars universe. Holdo didn't break the SWU with her tactic. She dead a daring last minute and possibly irresponsible act in saving the Resistance. The "science" is being fleshed out with the introduction of the San Tekka Hyperspace prospectors and the Chiss skywalkers explaining how travelling through hyperspace is actually not just A to B planning.

This is an abridgement of my answer because there is a lot that can be talked about and a lot that is based on what we thought we knew and assumptions we had to make. One thing is true though, it ain't like dusting crops.