I’m in the same boat as you. Used Textra and Mighty Text years back but want to try swapping back. Doesn’t seem like anything can compete with the robust features Messages has right now, unfortunately…
Oh shit, I missed that part in the post. Go into settings/mms/mms advanced and turn down the carrier send limit. It’s trying to send it in too big of chunks so your carrier blocks it
Open up textra itself like youre going to text someone but havent chosen who yet, hit the three dots in the top right, settings, under messaging is MMS. Carrier send limit is in there.