I did freelance work in 2023 and I'm now (2024) filing for my EIN just so I don't have to share my SSN with my clients. can use that EIN on the w9 I need to send them?

Yes, but you should have given them a W-9 previously.

This is the first year that they are my clients. They haven't filed their taxes so there hasn't been a reason to send that until now which is when they file their 2023 taxes.