Thanos can't control his structure at all. I don't know where this myth comes from. He can't change into a rabbit, nor can he stop himself from being vaporized from a very powerful being, or koed. He has bleed several times and didn't heal up instantly.
Longshot's powers backfires or stops working when he does evil or selfish things, or loses hope.
He basically alters the probability field and makes unlikely things actually happen.
He's a meta (and not a herald or above) because he can be selfish in his acts at times causing himself bad luck. He's not morally perfect.
If Thanos knows his powers then he can manipulate him into being selfish. But if he doesn't then the probability field would be in Longshot's favor.
Clearly youve never read Longshot saves the MU. In that series he went up against the split halves of the inbetweener. One was aĺ chaos kne all order. He was more effectual than Dr Strange. And he did a shit load more than the "odd trip up".
Even so he's gone up against a lot more than what you suggest. For one thing hes had a bettle with Juģernaught where Juggs couldnt even touch him. Also the "overweight spider guy" is a highly magical entity from an extra dimensional and extratemporal realm. Ye basically wxists outside of time and space and can manipulate both, has psionic power to the degree he has mentally subjugated an entire world.
Given the fact that he is also Longshot's creator (along with the creator of Spiral, Quark and other powered beings from mojoworld) and responsible for giving them their powers, the fact that Longshot handles him easily (even when he has amnesia and is unaware of his power and how to use it which is often as Mojo wipes him mind a lot) is quite a statement.
Which brings me to the point thT all the low showings you refer to are of a mind wiped Longshot who hasnt learned how to use his power (just instinct). A plot device used to keeo him low tier, because when he knows he defeats armies from Mojoworld single handed.
To do high end feats yes. He naturally has a good luck field around him which is why even when amnesiac he's almost impossible to hit. He also has a degree of instictual use (reflex action) which makes some slightly better luck.
The statement that he can oush his luck to far has it wrong also. Becasue he bends probability when he makes thinges that are extremely improbable occur that means a counter effect has to happen somewhere. For him makjng his luck that good someone else gets hit with really bad luck. The key example of this was a stunt woman he was sharing a flat with in Hollywood. His extreme changes of probabikty caused to to get killed. So its possible that a lot of memebers of the low.meta tier get killed by bad luck to balance out bis good.
The downside of his ability is he can never use it for personal gain, when he tries it just goes haywire.nif he uses it to try and get something for himself no go. For example when he was hungry and wanted to luck himslef being given a meal the food truck exploded.
On the mid meta tier I'd be more worried about the likes of Dr Occult with his disk that can redirect energy or Black Knight who's ebony blade does also. These two alone ca turn any attack Thanos makes right back at him.