Is it possible to automatically attach multiple contacts to a transaction so when users send an email out of the communication tab it's prefilled for the recipients? (these will change per-order)

Yes, I built a customization with UE script that does that, you can enter multiple email addresses on the customer record and it will put them into sales orders po's invoices, things like that.

I found this blog post yesterday, and was planning on experimenting with it when I have some time. blog.prolecto.com/2024/08/04/learn-how-to-automatically-populate-netsuites-email-composer-additional-recipients-sublist

Yup that is the basic idea, of course they don't know your custom fields and how your system is configured. You want the UE script to reference some custom field somewhere, and then probably a before submission reference that field and insert it in for you. There are some caveats to that though, for example if you change the email address on the customer record it won't go back and change the email address on SO's already created. For that functionality you're going to need another UE script on the customer record that iterates through open transactions.