Is there a marking (or a term) that means "go back to the dynamic played at before the last (de)crescendo"? I'd prefer to not specify a particular dynamic immediately before the crescendo (in order to re-state it immediately afterwards).
I think the answer is "no, there isn't a standard notation for this." I've never seen one, anyway. But dynamic marks intended for humans (i.e. not for computer playback) have traditionally relied on common-sense interpretation. For example If you have "ff" and the next 50 or 100 bars of music contain several short crescendo hairpins with gaps in between them, common sense would suggest that you didn't intend them all to be cumulative.
If the dynamic marks are for computer playback, you will have to be explicit about the dynamic levels anyway, even if you hide some of the marks.
There is nothing to stop you writing a one-off instruction in your preferred language to explain what you mean - but preferably use your native language, not a semi-literate translation into what you hope is Italian!
If you want to use this often in a piece, you could invent a different shape of "hairpin" and explain what it means at the start of the score. People have been inventing new "contemporary" notations for hundreds of years - that's part of the reason why "standard" music notation isn't always logical.
You don't have to state a dynamic immediately before the crescendo unless you want a change there. You DO need to state a dynamic after it though, otherwise how does the player know whether it's a crescendo to mp, f or ff?
Computer scoring programs, and the tricks required to get expressive playback out of them, have encouraged a generation of composers to over-notate. If you find yourself continually writing mini-crescendos and diminuendos, consider whether you're just confirming normal expressive phrasing. When a musician is given a dynamic, articulation or other instruction on EVERY note, he won't notice the ones that matter.