Hello fellas, hope I can be of some help (although do not take my advice as gospel!)
The main strengths of a Harad list are, as you mention, bows (and the 50% bow limit) and poison, along with lot's of undercosted troops with very good special rules.
Looking at your list, I'd recommend the following -
1) Watchers of Karna are one of the most "broken" units in the game. 2 attacks, f4, resistant to magic, poison with bow, s3+… If a bow line is what you're going for, take more of these guys if you can and use your 50% bow allocation.
2) Warriors of Karna upgrades in only worth it if you don't have more Watchers of Karna. Take watchers if you can!
3) Taskmaster might be a bit wasted in this list. His warband is a bow war band, whereas his special rule would be best supporting a combat warband within range of a hero likely to be using heroic actions (heroic strike, heroic combat, heroic move being the key ones here) He can't use the rule for his own heroic actions either.
4) Betrayer's masters of poisons is great for the Harad bow line, rerolling those 1s and 2s. Might be wasted on the 5 unit cavalry group, which will likely be separated from the other war bands in the game. Yes, you can then reroll your 1s and 2s with the lances but might be better placed elsewhere.
5) Some people might recommend that a banner at this points cost is less effective, but that really is your choice
Just a few musings from me, hope it helps a little!