So I've lost my 12-word seed but I still know the first letter of each word.
Can I make a script trying all possible combinations and recover my seed in a reasonable amount of time?
You only need the first 4 letters of a BIP39 wallet seed to be able to recover your coins safely.
It is impossible to know the correct wallet seed by knowing one letter, but knowing more words will reduce the search time, so try to remember more wrods by searching for the first letter using
https://github .com/bitcoin/bips/blob/master/bip-0039/bip-0039-wordlists.md
If you succeed in remembering 9 words, you will be left with (2048^3)/2, and after that you can use
https://github.com/gurnec/btcrecover/blob/master/docs/Seedrecover_Quick_Start_Guide.md if you do not remember the correct words.