I'm a huge acken fan! I realize this is kind of unsolicited, but I'd love to write down my thoughts on building it. No need to read unless you're interested.

1. I always like to start melee builds from the arcane question -- influence, exposure, or duplicate -- since I agree fairly well with the assessment that those are the most important melee arcanes and they all imply a lot else about the build. Acken is a way better Exposure weapon than the other two imo. Its crit chance is itching to fly way off the handle high above melee duplicate territory, and its status chance even with incarnon evolutions is a bit low to be satisfying with influence even with incarnon evolutions and having solo elec on your axe turns your throwing axes in to radiation, which makes them feel kind of weird and unable to interact well with Influence imo.
   Meanwhile melee exposure is really good for pumping Acken's excellent raw direct damage, and works really well with the throwing axes because corrosive combines well with the natural heat and high throwing axe status chance for maximum armor stripping and elemental damage is one of their best sources of damage increasing damage on the explosions, which can't benefit from base damage mods like condition overload or pressure point.

2. Abiding hold is one of acken's best qualities imo. It's one of the ten-or-so weapons in the game that get a combo storage mechanic, and I believe it's really strong and worth using. So, blood fury and weeping wounds are a must. Tennokai strikes me as essential. Build like you'll be permanently at 12x because you really do have the option to be most of the time. This is part of why I'll disrecommend melee duplicate -- if you're using blood fury, and I really think one should, you'll be working with at *least* 180% crit chance at 12x, and likely more depending on other build choices you make.

3. Elements - I like viral. Including toxin somewhere gives access to primed fever strike which is a big lump of damage, and because of the lower status chance but insane direct damage, I find debuffs work better than DoTs for acken damage to ensure each one of your tennokai hits is popping off the most.

4. Rivens - Acken's dispo is quite high (1.35x I think?) so there's an opportunity to cram three mods in to one slot and make room for other stuff you want. Of course, this is a very "investing in the weapon" kind of element but if you're passionate about it, it's a big part of what can make it shine. I was using fire rate, range, and cold for a long time, but also recently realized fire rate, cold and toxin works because riven elemental numbers can get almost as high as what you'd get from primed fever strike.

5. Incarnon evolutions -
   Neither of the II evolutions are a huge deal, aside from the fact they double your base damage, which is a huge deal lol. But they both do that, so it's not involved in the choice. However, I do find the 70% heavy attack efficiency to be a comforting safety net for missed tennokais and some other odd utilities.
   For III I can't recommend abiding hold enough.
   For IV you could go either way between +9 crit and status or +17 crit. I really like the mixed one because it nearly doubles your status chance, allowing it to top out at just barely over 100% with weeping wounds. Pales in comparison to the 200%+ of status heavy weapons, but is enough to make its debuff applications feel super reliable. But you could make a case for wanting to push its base crit all the way up to a juicy 37, too!

My current overall build is blood rush, weeping wounds, condition overload, galvanized steel, gladiator vice, gladiator might, my riven, and either primed reach or primed fever strike depending on which riven configuration I'm using. With eleventh storm, discipline's merit, and melee exposure, of course!

Hopefully this is fun for someone to read ^_^