i'm sure people in /r/slavelabor can migrate your stuff cheap-ish-ly, but if you want to do it for free/do-it-yourself, you can use Thunderbird to download all your emails from google workspace and upload them on the dreamhost email account.
btw "unlimited" emails are included in "Dreamhost Shared Unlimited", if you upgrade to unlimited, dreamhost should be able to host all your emails for... well, the current price is 10.99/month if you use the 3-year commitment, so 10.99/month