Lots of problems here...first of all, some of your modules in the build are overlapping really badly, at least in the display view. You need to go back to the build page, call up the "screenshot view" again and hit "refresh" to get the proper image up. Secondly, you can't mix Intellijel format tiles and "standard" format tiles in the same row, as they're not the same size. Third: even if Cwejman was still a working concern (all indications are that it's not), nothing in reality can excuse $600 for a stereo panner/VCA. I don't even care if the solder was custom-made by GOD, nothing justifies that. And lastly, this is a pretty small build, and small builds tend to never work out well if you try and build several subsystems into the same build. In this case, trying to jam drum modules in is something I'd discourage...not because they don't work, but because of the space they require, as well as the space everything else requires, means that the whole build has to be done by underequipping something or another. And you can see some of that here inasmuch as the only sequencer for your drum modules is the Steppy, and all you have for a "voice" source is a single complex oscillator. That there isn't necessarily bad in of itself, but other points are downright strange...such as using a stereo VCF when there's no other stereo modules in the synthesis path.
Since you've got a prebuilt system already, what I would suggest is to replicate that in an MG build first. Then, once that's done, expand the rack further so that you have build space that can incorporate new build activity, and then build into that. By having it AND the sketch system in the same place, you're far more likely to be able to notice how you can expand the Erica effectively. This wouldn't necessarily have to be a public build, either...just one that allows you to see your whole "modular picture" at the same time.
Now, as for the drums...quite honestly, just get a drum machine. For one thing, the expense of Eurorack modules IN GENERAL means that you can get a decent machine for far less than the percussion modules, sequencer, mixer, etc etc that a Eurorack implementation of the same sort of thing would cost. Secondly, they're machines specifically dedicated to beats and rhythm, not a general-purpose device that you have to kludge up a drum machine in. Unless you're willing to throw a lot of $$$ around and seriously mash the space available, you simply cannot build a subsystem (or full standalone system) that can do the same thing. Or a better way to put it might be like this: your Plonk alone costs $349, and a Behringer RD-9 costs...well, $349. And that's $349 with NO extra patchcables, NO panel space robbed, NO extra expense, etc. Yeah, I get it...I don't think much of (and try to think as little as possible about) Uli and his antics, but thus far his clones of synth unobtainium have been pretty spot-on, at least as far as the ones I've encountered. Even so, diving head-first into the expensive stuff sometimes works well, but sometimes it's just a flex that'll come back to haunt your build later on.