That must be frustrating. If you’ve already worked with them a couple of times and they like you and you like them and have built a relationship, it’s still 100% fine to say “hey, I need 50% up front, 50% at the end. I love working with you guys, and I’m happy we can continue to work together regularly.”
You don’t HAVE to explain yourself or justify it. You can simply say “This is my policy for all clients.” Or you can choose to explain further and how you enjoy working with them but an 8-week delay in payment is too much for you.