I didn't realize the POV changed between the two. That's odd because I always heard that Dune Messiah was originally part of Dune but the publisher wanted to separate them. Maybe Herbert reworked it before publication. Anyway, maybe you could read it now after you've spent some time away from the first book so the POV shift won't seem as radical.
Actually, it's my understanding
Dune is a combination, rather than that
Dune Messiah was split out.
Dune World was serialized in 1963-4 (
Analog), and
The Prophet of Dune was serialized in 1965 (
Analog) and both were combined as
Dune (1965, Chilton).
Dune Messiah wasn't serialized until 1969 (
Galaxy) and an even longer gap occurred before 1976's
Children of Dune.
I endured a first read of
God Emperor (1981) and
Heretics (1984) but couldn't make it to
Chapterhouse (1985) and only possess the first three at this point. I could see that being reduced to just the one if I ever get around to re-reading them, though all three may survive. The way I remember it, IMO, #1 resolved satisfactorily but made me willing to try #2 and it ended in a completely non-standalone fashion that gave me the option of quitting in midair or going on to #3. I went on to #3 and it wasn't bad and more or less ended and I could have and should have stopped right there. But it certainly left room for #4 and I went on to that but it wasn't good (or, as I said, needed). No idea why I read #5 and it was pretty completely useless. So I dumped #4 & #5, never did read #6, and feel ambivalent about #2 & #3. IOW, you're probably okay, Brian.