If the sender isn’t using Signal the text is unencrypted like standard plain text SMS I believe.
The cleanest solution would be to use a G.Voice alternative for calls and plain text messaging. Port my Tmob number to this service and take a native number from Tmob and not give it out.
Or… Get VoLTE functioning on these phones 
(I know easier said than done. 
)
If anyone out there has a tried and true alternative to G.Voice please share. (FOSS preferred of course)