There may be a loophole for you to exploit. Back in the day, there was a 1,000 character limit on the initial bid. BUT; once you typed (and paid credits to send that bid), you could do a follow up message. And, on the follow up message I think you could type to infinity.
Detailed job postings often require detailed bids. Many of my bids push the 1,500 character limit. What you can try is something like this: Hi Job Poster: I'm Mike. Welcome to Thumbtack and thanks for selecting me from the list. More in a bit; Mike.
then.... on your next message....which has already been typed out so it can be quickly sent after the first one.
Hi again Job Poster: Thumbtack limits the length of our initial reply. Once we do that, I believe we can type to infinity.
and then type your lengthy detailed message.
I was doing this quite often when the 1,000 character limit was in force. These days, I can usually make 1,500 characters work (which is the limit on initial messages from the "opportunities" tab.