Oh, boy...well, first up, that version of Maths is very discontinued for a long time now. Next, there's sort of a lack of oscillators here, and you need several to build up drone spectra. Look at some prebuilt drone devices such as the Grendel line...these have at least two VCOs, and while the Benjolin has those, that module is more of a 'random source' device. Drone synths also don't usually need a keyboard controller, either, and losing that will free up a lot of space for stuff you do need.
I'd kinda do this differently...lemme think about this for a hot minute...
(time passes...not much)
OK, now THIS is a serious drone instrument:
Top, left to right: a set of lo-fi VCOs with a restricted frequency range to build up low end. Then a west-coast-type dual VCO that allows complex cross-modulated timbral textures. A Telharmonic...this gives you the ability to sweep through additive harmonics and a few other nice-and/or-nasty tricks. CVable Fold with subosc divider, and then four VCAs to control different source levels and mix those down to the VCF input.
Bottom: Power supply...I went with an Arturia RackBrute here, because you get 4 more hp per row, a beefy power supply for the size, portability, and it's stoopid-cheap at $359. A dual clock with logic to cause strange rhythmic behavior. Then TWO Maths (current model) because you want lots of complicated modulation for everything in the audio chain, as well as to cascade into each other for somewhat unpredictable modulation behavior. The Doepfer VCF is not only a redux of the Korg MS-20 Sallen-Key pair, it also includes an insert in its resonance path, into which you can add the Chronoblob delay next to it. Or send the audio to the Chronoblob and put the filter in ITS feedback path insert. Either is good! After that, I put in a third-party build of the Clouds module so you can granularly tamper with the audio stream. The output is a Bastl Ciao!, which also allows you to put two different stereo streams in, gives you a headphone amp, and balanced 1/4" outs.
Anyway, that's how I would do this sort of thing...given that I've done a lot with drone techniques, this is sort of a present-day Eurorack build of some of those ideas, but beefed-up with the newer modules Euro has to offer. So it's a decent example to start with, since the overall architecture is pretty proven, but definitely tinker with the idea some more.
(FYI, if the rack looks incomplete, click on it to go to the actual image. MG seems to be beefin' on putting the completed image up at present. Grr...)