I live in China, where a lot of the stuff is made. So YMMV. However, it is amazing how prices vary so widely here. Some stuff is dirt cheap, for example I get brand new Pentax lenses at prices close to the low range of used prices in the US. And then I might have to pay double the US price for a smartphone that is actually made in China…
Anyway, to get back on-topic. Pentaxforums.com has several threads extolling the virtues of the Yongnuo 560 mk II and mk III flashguns. They are not as whizz-bang as Nikon guns apparently, and they are not fully automatic – they may even be manual only for all I know. BUT, they seem to be dead cheap, and I think manual only is a good idea for studio work anyway. And while I was checking them out on the Chinese amazon site, I found some nice portable LED panels made by Yongnuo that could be a nice low-heat way to provide continuous light. One sec, I will check US amazon…. Yep, over 20 pages of Yongnuo stuff to drool over, I must not look, I must not look….. 50 dollars for a small panel, but don’t forget the battery pack costs too, and may well be heavy.
Such panels are probably nicer for doing baby shots, because they won’t be so hot and disturbing. Sometimes you might need the heat to keep the baby warm of course. A varied array of light sources may create havoc with the white balance, you are gonna need to be careful. Or b&w only. The rest of your gear sounds very useful for your needs, at the moment, but maybe a nice 40 mm f2.8 at $277 or the 35mm 1.8 at $199 might be a useful lens to add to your collection. But stand well back using these lenses, you don’t want the dreaded bignose effect to show up.
Glaring omissions from your list include a really good tripod, and, sad to say, the most expensive, and in my opinion the most necessary addition – at least one extra camera body. If you still have the D90, splendid. 800E on the tripod, all set up in advance, D90 with 35mm f1.8 around your neck. You look the real deal, and you can prance around impressing the hell out of mom, swooping back and forth to take candids as you go. BTW at this point we should think about the new Sigma Art lenses. They seem to be built using the big-SUV ethos, so they are enormous and heavy, but they are cheap and are getting a good rep, and it seems that those not in the know equate enormity with professionalism, so the customer might swoon over the fact that you are blessing them with all your supergear. (I, on the other hand, love my tiny Pentax lenses, I carry 4 primes and a fisheye zoom everywhere atm, my gear weighs about 3 kilos in total)
And so on. And now you meet a real stumbling block. You may well be thinking of going semi-pro to help you buy that item of gear you dream of owning. And here we are, proposing that you have to spend even more money on gear you will need to get to become semi-pro, BEFORE you can start saving up for that dream item… Sigh.