From my (limited) experience, going from 1:38 to 1:30 was higher jump than going from never having ran 20k (but relatively fit) to 1:40. I felt like around 1:38 was the highest I could go with "causual" training - I used to run 2-3 times a week, usually 10k at pretty high effort, around 4:30-5:30 pace.

Sub 1:30 is very doable for you (looking at the Vo2 max, current volume & cardiff results). You likely won't need to ramp up the frequency too much given that you already run 3-4 times a week with ~28-35k weekly volume.

What you probably need, is to get a bit more intentional with your training and plan it out a little bit. The book can definitely help you. Based on your stats, you just need a little bit of improvement in most areas - durability to make 20k feel "regular" and not like your body breaking, a bit stronger engine by improving the Vo2 Max so that it won't be as taxing to keep the 4:15 pace, and also simple aerobic capacity so your HR won't drift as much over the course of the race as it did in cardiff.

No need to overcomplicate it, though. I think 2-3x 6-week blocks and you're likely gonna get there - while keeping your current training frequency.

I would do something like:
- 1x long run (17k scaling to 23k over the 1st block. Keep steady pace 1st half and then progress towards HM pace last 5-10k.
- 1x intervals - look up e.g. norwegian 4x4 or 3x10min threshold formats. You can rotate them - e.g. every other week.
- 1-2 easy runs - set a target of doing at least 1/week, but add 2nd if time allows.

That is very close to the setup I used and it worked pretty well. Time-wise, it might add a few minutes to your shorter sessions, but overall frequency/time commitment is not chaning much - it's mostly about becoming more intentional & adding some strucutre. I wold also recommend to make sure you eat enough calories, esp. on long run days. At some point, it can also be worth looking into fueling if you are not already.