Misty Step, a 2nd level spell, allows you to teleport up to 30'Rulings not Rules in action!
Swarmkeeper, my reservation about Thunder Step isn't that I think casters should be immune to their own spells- there are specific spells and abilities to allow for that. It comes down to the fact that if you can use Thunder Step in a manner where you would be caught in the blast yourself, the spell becomes hilariously bad compared to more efficient, lower level spells. You're basically trading a 3rd level slot to do something you can do with 2 2nd level slots. There are times when that's advantageous, to be sure, but imagine how less useful Misty Step would be if it said "you can teleport up to 30', but no less than 15'".
Thunderstep, a 3rd level spell, allows you and one other ally, to teleport up to 90'... and leave a damaging blast in your wake
I'd say the scaling works well here.
Also, point of order: as you can't cast two 2nd level spells on the same turn, I don't follow your argument that casting, say, Misty Step then Shatter over two turns is better than casting the single 3rd level spell Thunderstep in one turn.
It's a valid interpretation, but it would have been trivial for WotC to say "oh btw, instantaneous effects cannot be reacted to unless otherwise stated". Instead, they printed things like Shield and Counterspell, which do interact with instantaneous effects, and never bothered to say if they are the exception or the rule.
Shield admittedly is a bit of an oddball but spells out the very specific requirements to bypass the instantaneous effects of certain spell attacks: it can be cast as a reaction: "when you are hit by an attack or targeted by the magic missile spell".
Counterspell is specifically used when you see someone casting a spell, not when the spell takes effect. It doesn't matter if the duration of the spell is instantaneous, the Counterspell is trying to interrupt it while it is being cast (so during the action, bonus action, or reaction time needed to cast the spell).