What a discipline it is to write a hymn!
I've never forced myself to do it; it's much easier to check others' works for errors than it is to follow part-writing rules from scratch (hitting brick walls with parallels, etc.) and still end up with what you want

. So, first off, bravo for choosing this battle.
Your overall 4-phrase structure is good. The 3rd phrase provides the peak, and the use of the same beginning for the other three phrases ties the hymn together. Rhythmic similarity (half note, 2 quarters) at the start of each phrase also creates continuity. Good work.
My suggestions center around the "balance" of sound created by your use of doubling and inversions (ie, which chord note is in the bass), as well as some chord progression. I think re-considering a few spots will either confirm further in your mind what you already have or possibly help you see some more enjoyable alternatives.
(**PLEASE don't be overwhelmed. I've tried to explain things precisely, so you can undesrtand what I mean, but the comments boil down to relatively few very specific changes. Overall, great job.)
1. Parallels:
-ms. 4: tenor and sopr. have double 8vas on beats 1 and 2. As I suggest for this measure under #3, holding the lower 3 voices for a half note will avoid this motion.
2. Inversion (bass note) and doubling:
-ms. 15, 3rd beat: with the Bb in the bass, this comes across 2 ways to the ears: unsatisfying as a vi (because my ear "wants" the g in the bass), or a little "jazzy" (because a Bb in the bass sounds like a IV with an added 6

). To put it simply: if you want the iv, I'd put the g in the bass.
-ms. 16: you doubled the root and 5th with no 3rd: sounds empty. If you're going for the "empty" sound, you would want to write more of the hymn like that. Right now, it sticks out as very different.
-ms. 18: you put both the tenor and alto on the suspension. I believe you're never supposed to use 2 suspensions in 4 part style; rather, you'd want to double another chord note, and I'd suggest changing the tenor from F to G.
3. Progressions:
-ms. 4: IV-->iv-->V is a weak progression. You could just omit the iv chord on beat 2 and have IV last for 2 beats, sustaining the voices underneath for a half-note (the brief dissonance would be fine).
-the 7th in measure 8 is worth exploiting. It will sound the most "intentional" if you keep the bass voice on the root: Bb. To do thiat, I suggest you use this alternative progression for the 2nd phrase starting on the last half of ms. 7: I vi IV Vsus (changes every 2 beats).
-ms. 12-13: you move from vi to ii back to vi. I would avoid this, because 1) it goes back and forth, which is a weak progression (normal chord progression = vi-->IV-->ii-->V-->I), and it also sets up another vi-->V motion. Here's a possibility: vi IV ii Vsus (this will create 9th chord on the 1st beat of 13--may be out of character

).
4. Other notes:
-Some ideas for what happens at ms. 2, beat 3:
The C in the bass makes this a "cadential I6/4 chord," which (you probably already know this) means it usually functions in a cadence, preparing a V-->I motion. It's up to you, but I wouldn't use this inversion of the I chord without following with a V. So, here's some options:
Option #1: Since this same spot (ms. 3) occurs 3 times in the hymn, you can afford to vary it a bit. I would suggest moving down a 4th in the bass to a low F (on bt. 3) the first time. This is a more "settled" sound, and since the piece doesn't have momentum yet, ms. 3 is a good spot for "settled, solid" sound.
Option #2: Then, if you do my 1st suggestion, you might try taking soprano and alto down to G and E respectively on the 4th beat, delaying the bass movement to the B until the last 8th of the bar. This accomplishes both harmonic pull (V7) and rhythmic "pull" for the 2nd phrase.
**As a note, the motion of the C-->B in the bass really demands the V chord in my opinion, because 1) it clashes with the upper voices and 2) the previous I6/4 chord already implies that V is coming.
-For the ending, I (opinion here...) prefer ending on the tonic in the soprano, at least the last time through. I would at least suggest the other voices move down. If you move all voices to a lower chord note, you get a nice "relaxed-sounding" final chord.