Er, full disclosure, I've never swum faster than 1:07 in an IM despite being capable of a 6:03 400m, due to the fact that I really really suck at swimming. Thankfully, I can usually make it up on the bike and run!
6.03 for 400 means you clearly dont suck at swimming! I can probably manage a 5.20 or something, but trying to sustain a pace below 1.30/100 for the full distance is proving a challenge so far (plus having to bike and run after it means you need to be able to comfortably do it..!)
There seems to be this plateau at that pace for me which I can't get past, and I'm not sure if it's more volume, intervals, strength, OW skills, or technique thats needed (or probably a combo of all of them).
If you can swim 5:20 and this doesn't represent a significant falloff from your 200m pace, then I'd suggest it is volume.
Work out your pace per 100m. (Do 10x100m off 10 seconds rest, then work out your average pace across the swimming portions. You only need a stopwatch for this, you can use the pool clock for rest intervals)
Get used to knowing that if you swim 100m and come in +5 seconds, you're swimming a bit below threshold.
Create swim sets.
An example might be you warm up with 400m. 10x50m drill out, swim back (assuming you're in a 25m pool). Main set of 400, 300, 200, 100 at threshold plus 5,4,3,2 with rest to the top of the next minute or whatever is appropriate*. Cool down.
Keep it simple enough to remember, do one or two key drills per session and read and remember what you're focusing on for that drill. The danger flicking through a dozen drills in a session is you never get enough time or mental capacity to really think about what you're doing. Work on body rotation one day, breathing another, catch another, etc. Swim often enough and they come together on their own.
*I know this looks complicated but for example: I swim at 1:25/100m, so 100m at 'pace plus 5 seconds' will take me from "top of the clock" to 30 seconds.
If I'm doing 400m, that's 4 of them... leave on "red top", 100m split will be at "red 30" (which is usually black top). If I'm ahead of pace, I might see red 27 when I turn. I know to back off a bit. If I see 32, I give myself a leather suppository. By the next 100m, I will be looking for "red top" again.
Yes, other numbers are less convenient, but even if one swims at, say, 2:01/100m and you're swimming as threshold +3 seconds, you know you'll come in at (1+3=4) 4 past, 8 past, 12 past, 16 past. You can rest until the next hand is at the top and repeat.
A bit harsh TE! You could say the same for running. I don't use a Garmin but once wished I had one when swimming against a current. And instant feedback is a good learning aid when trying different tempo sets or stroke mechanics. Just saying....
I don't think it was harsh.
If you're swimming without a watch you need to know what to do. If you're swimming against a current then a GPS will be meaningless (and it often is anyway when swimming, accuracy can be very poor), it'll only tell you that speed is slow for high perceived exertion. Which you knew because there was a current.
My best runs have come on tough courses when I've totally disregarded any "pace" feedback from a watch. And I'm probably the biggest data geek around. The body knows when to back off and when to push harder - chasing a running pace up and down gentle grades will blow you up far more quickly than just doing what feels consistent.
The caveat is...you have to train for it. Go out and get used to holding that pace, whatever it is. Applies for swimming, biking and running. It's not complicated!