It's a shame to see that the same problem this species had since its inception is still present: DCSS already favours broad skill training. As a human, level 7+ spells and getting the heaviest weapons to min delay are
already bad investments. The optimal skill training for this species is almost exactly the same as it is for a human. I went ahead and
played one to try and show this; I did what I believe is optimal skill training for Bu (except for when I messed up and accidentally overtrained a skill; just mentally truncate the fractional parts of all the skills to see what it would have looked like if I hadn't messed up), and I think I would have pretty much done the exact same skill training with Hu. I didn't optimize my equipment - the boots of running were better than the slaying boots - but that's unimportant to the topic.
Basically Bu/Gn starts as a better human and ends as something almost identical to human, unless you suck at the game in an extremely specific way (thinking high skills are a good idea, rather than something to do for fun/convenience). The only significant differences from human that I noticed after entering Lair were that I actually had a reason to train shields, and that my melee was better because I had a free OP aux attack for no good reason.
I have heard two entirely conflicting claims about the intended design of the species. Floodkiller said it's intended to be a "generalist species" that can pick up everything easily, but not excel at any one thing easily. Brannock says it's intended to be easy early and hard later. Obviously, it accomplishes neither at the moment.
If it's supposed to be a generalist speciesI think this design has promise but you cannot accomplish it with the skill restrictions. Characters in DCSS don't excel at a specialty based on skills, they excel at a specialty based on items. Demon weapons and properly branded quick blades are as good as bardiches and exec axes and triple swords. Maybe even better; definitely not worse. High weapon skills are just a substitute for when you don't find a demon weapon, and high spell skills are never a good idea until extended; all the worthwhile spells are level 6 and lower.
I still think the only reasonable way to accomplish this is to lock their str, int, and dex at low values (no higher than 5), make it so you can't change their stats with items (wearing a str+6 or str-6 ring just does nothing), and give them high aptitudes with no aptitude falloff. This lets them pick up weapons very quickly, but because of their negative str multiplier they will never get close to the damage output of a melee character of another species once that other species has reached min delay; and it lets them pick up spells very quickly but they will never have great spellpower. Yes, I know you tried this version and "nobody liked it", but imo it would be a massive improvement over the diminishing aptitudes version.
If it's supposed to be easy early and hard laterWell first of all that's a terrible stupid design, but fine, if you really want that, you can do it with the skill gimmick. You just need to be much more aggressive. Training a skill to 10 with Bu/Gn should cost more xp than training a skill to 27 with a human. It'll remain godly in the early game and actually be noticeably worse than Hu later, in that case.
*Note about the game: it is not normal for training multiple weapon skills to be optimal; what happened in that game was that I started with axes then found a polearm good enough that it was optimal to switch to training polearms skill (the pain spear), then later found a mace that was good enough that it was optimal to switch to training maces skill (the elec dwhip).
edit: Another option is to change the global skill cost tables (or rebalance weapons and spells) to make specialization more skill-dependent and narrow skill training more viable. This may be desirable anyway, but isn't going to happen fast, so if you want to go with that plan there's no way this species will be ready in time for 0.20.