I've finally started playing around with the BFG Seraphim this past week, and although some of the initial ideas in the OP are still present I've done a completely different build.
You see, after having a bargaining tree up to level 100 and a Niob smith I was basically handpicking all the best possible gear/weaponry for my toons, and from level 1 for that matter. I started an Inquisitor build based mostly on Gruesome Inquisition but with some tinkering of the other aspects (mainly the permanent and temporary buffs) and he's so bloody offensive that it stopped being challenging quite fast. Dual wielding with a combo of Clustering Maelstrom+Ruthless Mutilation followed by a combo of Ruthless Mutilation+Callous Execution makes the ground tilt in red... So basically I thought to myself how feasible it would be to just play with all the general skills that would make my toon totally independent and autonomous, namely Enhanced Perception, Bargaining and Blacksmith. And so the adaptation towards an "all-in-one" BFG Seraphim came into place.
The basic template is more or less like this:
2 Enhanced Perception
3 Ranged Weapons
5 Tactics Lore
8 Armor Lore
12 Bargaining
18 Concentration
25 Exalted Warrior Focus
35 Revered Technology Focus
50 Toughness
65 Blacksmith
General skills are first in line for mastery, then Armor Lore and Toughness. Tactics Lore and both focus skills have enough points to modify Battle Stance, Soul Hammer, Pelting Strikes, BFG and Divine Protection. I'm skipping Revered Lore so no point in using Archangel's Wrath because Soul Hammer's damage trumps it. I also have no interest in using three buffs, only one point in Concentration. At first I thought this would be spreading too thin (and to a certain point I still do) but eventually the build started shaping up to be better. With lower regen times and higher ranking CAs I can wipe the floor of the Orc Cave in sometimes as little as two snaps of the SH+PS combo. I just purchased a Volcanic Tiger special mount and it covers some of the weaknesses like low HP and low shield energy. So far it's been fun and challenging in terms of approach and micromanagement. Bosses are actually easy because the BFG boost to Ranged Weapons is static, so I can just move into a different weapon slot with a throw weapon that has "+X% LL" and it'll still have the same benefits, just not the boost to Tactics Lore since that one is on the actual weapon of the BFG. So bosses are covered and crowd control is as well, mostly via gaming style instead of Area of Effect CAs. Either kill from afar or drop Divine Protection while rounding up mobs and then discharge rapid fire with the multi-hit combo. It's actually very effective since the weapon will switch between enemies very fast when casting Pelting Strikes, assuming they're in close range.
Now if I can find the other five pieces of Genesis Siderea (I actually have two already but one is level 135...) I can have an always leeching BFG. Plus, at higher difficulties the BFG damage combined with the "+X% LL" bonus from the set should really help in keeping my toon alive! We'll see.