I have only just started to use the SMS sending feature and texts look like this when delivered:

DELAYED DELIVERY[2022.12.12 18:31:8]:

Test message

NOW:

Test message

I tried setting the IM/SMS mode to VOIP SMS , but no change. I tried Zoiper (which I don't like) and it sent the message just as I expected, so I think the issue is with MizuDroid not my VOIP provider. Is there any way to stop this DELAYED DELIVERY message?

For some reason the message was marked as undelivered. Maybe your server haven't sent a 200 OK answer or there was some other problem.
In this case if the "Offline chat" feature is activated, MizuDroid will try to resend it later when possible (before the next message or with the next successful registration)