I have my whole comic written already, and have for quite some time...
Sometimes I get bored with my comic as a whole and reread my script to remind myself where the story is going next and I'm always surprised at how new the story feels after.
I wrote the majority of it in quick bursts over a few weeks before I could lose the idea and just stuck it in script form in google docs. This meant most of it was a complete mess from the get go, but at least it was on paper! I asked my friend to help me work on it from there and it pushed it from barely coherent to something I could really invest myself in.
I knew what events needed to happen and had a vague idea of most of the conversations that needed to happen and filled it in by section until it read smoothly. Sometimes I jumped around. If you know how it needs to start and where it needs to end you've already won the hardest battle!!
My script roughly looks like this:
Chapter summary paragraph
[panel description]
person A: dialogue
person B: dialogue
[panel description]
And so on, for the entire story. I also color code the sections, one color for panel descriptions, another for dialogue, another for summaries, and another color to highlight sections ive already finished and posted. This way I can see where I'm at without even reading, and I can scroll really quickly to specific sections.
Just about every instance of dialogue changes in some way when I actually draw it out. Sometimes whole lines get cut because once I get to the panels I see that they're actually redundant, or just too wordy, and I need to make them more concise. The script for me isn't the "final copy" and in this way I'm not locked in to anything except the big details.
I would say my best advice is to nail down the big details, because consistency matters! But don't worry as much about every line of dialogue being perfect before you ever start drawing, because what works beautifully as a paragraph will be a train wreck of speech bubbles.
My second best advice is to ditch grammar, spelling, capitalization, punctuation-- anything that slows you down while you're writing, throw it away until all of your ideas are on paper. As long as you can read it now you can fix it later. If your script isn't hot steaming garbage at first you've spent too long on formalities and not enough time on the story lol
When you do revise it I still don't feel like the script needs to be pristine. My script is kind of awful, at least in the sense that it's written precisely how I'd message my friends over discord and I didn't give a shit about apostrophes or spelling my own characters names correctly.
As long as your finished pages have the finer details the script can wallow in linguistic sin.
Lastly, if you're feeling stuck, or bored, or like you've read your script so many times it doesn't even look like words anymore... Bring a friend into the mix. Have someone else read it, let them give you notes about what they liked, what's confusing, what's absolutely awful. I'm certain it will help!