Curious to know about different pace strategies for HM. I ran one last year in 2-3s increments (ascending pace) and felt great but I didn't go all out (1:46 finish time). I'm training for a HM in May and shooting for sub 1:38 but not sure if same pace strategy would work given the big jump in overall avg. pace

it all depends on your training. if you are training to your limit (the pace you trained for), keep a constant pace as much as possible and if you have enough energies try to push more in the last 3 km.
if you are still not optimised you can do a progression and then know that you under-trained. 😉

a few seconds gap between the first half and the second is good. but if the gap is huge (let's say you start that 5:00/km and finish 4:30/km) it means that you could have done better and you were too conservative in the beginning.

Noted, thanks Alberto. Why would doing a progression imply being undertrained?