Definitely don’t pick QS1 this late in ranks.
With Mergen you might have difficulty with focusing on upping weapons since you need to have a good bow for Mergen skills then a good xbow + dagger for backstab. Not sure if Mergen skills can be used while burrowed either. You can capture Arrow Sprinkle though.
With Hacka, you can use Backstab even when mounted as well as the sword you’re equipped with. Feint, Capture and Lachry will still dismount you though but with practice, a good ping and some key remapping you should be able to remount with ease after using these skills. Burrow hates Hackas.
I’ve actually seen quite a few Hackapells do well in PVP (Telsiai) with a circle in Scout. Quite an interesting build really. Their approach is to turn off pet (they use “Mount’er Good” addon for this), use cloak, get near you, turn on pet, debuff (Lachry, Feint, etc.), mount then burst you down with damage combos. Might seem complicated on paper but like with other good combos out there, practice makes perfect.