To make it short:
(well, as short as it gets)

1.) Different sizes in lvls
- ranging from 4-5 to 8-10 minutes on around WZ 160 for me currently
=> at higher levels, players ARE REQUIRED to accept and surrender WZ mission until they get small maps (!) (!) (!)
Please, Neocore, tell me that this is not intentional.

2.) Dull and boring WZ "levels"
- there is no variation besides level size
- highly discourages build diversity
- feels like being a janitor cleaning rooms instead of playing an ARPG
As an example, if the goal were to just destroy the gate, and the gates had a bit more HP (2-3 times), there would be different ways to go about it, by either clearing out the mobs first bit by bit (dps-focused build / play style)), or taking the hits for a few seconds and just breaking the gates, then escape (tank + burst dmg focused builds), and ofc other builds/ways that I miss here as I can't think of everything.
Having even different types of missions (to CHOSE from, always!) would further encourage build diversity.

3.)
The 15 minutes timer
- rather unrewarding, put punishing
- think of instead giving various bonuses for completing in certain times, or, ...one might ....might even think of other side objectives... no?! ...right! that would be too insane! better not do that!

4.)
Story
Seriously, no voice, not even at mission start?
All "story" there is, is a BADLY FORMATTED WALL OF TEXT?!?!
Come on! I know you can do better, or well, I hope you still can.

5.)
Making it more interesting
Actually, scratch the "more", just make it somewhat interesting.
I could think of random, interesting things to spawn/happen in WZ missions, like mini-bosses, rare spawns with rare loot, even simple chests... (new) side quests .... something....

6.)
Scaling upwards
Right now, dmg received is not an issue, and ..."difficulty" ... "scales" by the -xx% dmg dealt modifier steadily creeping upwards, whereas I fully expect it to reach 100% at some point,
and some steps, like from 97 to 98 are quite steep I have heard, there are things known as "digits"... you may want to try that,
or better even, revamp this "scaling" completely with something that makes more sense.

7.) Nemesis
Well, it's there, it's a start, but right now it's the antithesis of an interesting boss battle,
I did lvl 13 and was sitting in his face for 2 minutes without moving until it died, that's not a boss battle,
that's a huge ass bullet sponge.
Seriously, remove some of those obviously arbitrarily chosen and untested HP/shield-HP and give that boss some meaningful mechanics instead, just look at ....PoE or any other halfway successful ARPG and just copy the good stuff, everyone does that, and as long as you gracefully coat it in WH40k-flavour, your customers will be super happy about it.