There is a simple workaround if you just want to enlarge all text on iOS Messages.
In Settings > Control Center, add the widget Text Size. Then switch to Messages and invoke that widget from CC. Make sure at the bottom to choose Messages Only and then move the dial further up. That will increase all text size within Messages.
My issue is that my Messages test size is fine the way it is for me, except for teeny tiny emoji. How hard would it be to implement a double-tap to enlarge or a long press to get a loupe view? Really Apple. Usability/accessibility and all.