Always upgrade the grinder first. But I would suggest to consider the HX you're looking at and if you're not really happy with it, just wait until you find one you know you'll like on the forums. In the mean-time enjoy just about any other type of coffee prep much better with your K10. Bitterness should be completely gone in vac pot or press, so I see no need to rush espresso.

You might also consider a MyPressi Twist to fill in your espresso fixes. Though I don't think it's a great replacement for a machine, it might tide you over for a while and be worth the money as I hear it pulls a great shot, especially with the big conical.

Personally I'll be doing the same, upgrading to the K10, and then seeing which machine I want to get at my leisure. Of course I have a small lever for the tide-over period...but it's really true that the grinder is a HUGE portion of espresso. I went from a Mazzer SJ to a Kyocera hand-grinder and my shots went from very repeatable to disappointing most times and surprisingly OK very rarely. And this is with a PID to control temp.

Go with the grinder, you won't be disappointed.

Also, in terms of bitterness, you are going to find that temperature / beans / roast have much more effect on bitterness than the grinder.

JMO, but I disagree. I've roasted all sorts of home roast experiments, and although very dark roasts will taste ashy, I think the biggest contribution to bitterness is overextraction. The large conical will help with that more than just about anything else. But maybe I'm wrong...