Interesting rack. I'm going off of the version of your rack you modified yesterday, not the one pictured (slight differences). You have the basics covered. The only thing I could really critique is a lack of a dedicated LFO source. Granted the Pam's can generate them and the STO can get down to 8Hz... but 8Hz is kind of fast for an LFO.
I would hold off on the STO, Threshold, and Databender for a second round of purchasing. Always leave yourself some room to change your mind or fix some deficiency you've overlooked. If you populate a rack completely, you have no wiggle room and will either need to buy another rack, or remove/sell off some modules to make space for what's needed.
It's a bit of a broken record, but I would try to work in some sort of multi-function module. A Disting EX is a great choice since it includes a lot of great programs. The Disting Mk4 is okay since it's only 4HP. But the interface can be a little frustrating.
Check out the 1010 Music BitBox Mk2. It's 5HP larger than the Salmple and about $100 more. But it'll offer you twice as many trigger inputs (16) and having a nice big GUI is helpful (for me at least). Additionally it has a 3.5mm MIDI input. You should be able to patch it up via MIDI to your Keystep Pro to save using all of your gate outputs to trigger samples. It can also live loop record if you send it the proper clocks. The Salmple might be your thing. But at least know what else is out there.
The QPAS is a stereo filter. So to get the most out of it, you have to send it a stereo signal. Blades is a dual filter, meaning you get two independent multi-mode filters. You can use one for left and the other for right if you have a stereo signal. Unless you're planning on doing a lot of stereo work, I'd go with Blades.