Yes, Manis being mono is normal, all Noise Engineering voices are mono I think. You can always "stereo" a signal later, with effects for example.
For LFOs suggested by Ronin, I suggest you start by exploring the Hermod. It's got plenty of functions, one of those is LFO's. Always start looking at modules you already have to achieve a goal you may have, this is how you learn your instrument !
For filter, it's true that Manis has in own integrated LPF and Plaits can definitely be used without a filter especially when you use the trig input thanks to that LPG simulation, but a nice multi-mode filter can expand the sound palette on offer quite well nonetheless. For example, someone put the Dual Dagger from Shakmat Modularon my radar recently, relatively cheap and small but quite powerful it seems, and it can help with mono-to-interesting-stereo duties too.
Jb is quite right about the Blackhole DSP vs FX Aid. The latter is a no-brainer so if space is an issue, you have your solution right there. And in fact, there is also the question of the Black HADSR, 12HP for a single enveloppe is not something I would do in a space limited rack. If you absolutely need ADSR (I found out some time ago that for me, that was coming from habit more than need, I'm very much an AD guy now) you could have a look at Quadigy from Klavis, still ADSR but 4 of them, presets and other niceties, for only 2HP more.
Finally, I suggest going through Math's illustrated manual, a must try in my honest opinion ;-)
--- Voltage control all the things ---