You can’t send MMS messages over Wi-Fi. CAN’T. It won’t work.
And iOS messaging automatically converts HEIF to JPG when sending via MMS. And even if it didn’t that wouldn’t prevent sending, it would just require the recipient to have a device that supports the HEIF standard that has been around since 2017, but the message would still be sent. What might prevent sending is if the photo is too large to send via MMS. Then taking a screenshot (which is automatically low resolution, small size) would work but would actually be masking the problem.