ES9 is a DC-coupled audio interface - it allows you to send both audio and cv to and from your computer - it can be controlled by midi to some degree - so that it can also be used as a mixer - I don't know if it can be used as an interface and a mixer simultaneously - you would have to read the latest manual, which is available on the Expert Sleepers website

if you want to bring external instruments up to modular levels then you will need some sort of pre-amp module - or some vcas will also do this - the intellijel quad you have racked, for instance - but you may find you are using multiple channels to get enough gain - a mutable veils clone would be better as it has a lot more gain on each channel (if you want to use a vca for this) - personally I would get a dedicated input module - but be sure that you buy what you want by doing research and thinking about how many channels you want and what other features - envelope follower, gate extractor, pitch follower are all possible - and are all available in other modules - examples would be doepfer (cheap and cheerful and a bit gritty), befaco (more expensive but clean) and the sonicsmith ev1 converter (which adds pitch following amongst other thing) nb all of those are mono for inputting guitar/bass and
possibly vocals

other points: the Pams you have racked is the original - which was discontinued years ago - I would at least get a Pamela's New Workout (used - as again discontinued) or a Pamela's Pro Workout - the latest version which adds quite a few features...

do you need scales, the midi to cv module, pams and an es9 for sequencing? probably not all of them...

using es9, pams and the midi to cv module as sources of pitch - they will all be already quantized - this makes scales redundant (& I intensely dislike this word in regard to modules)

keep pams (one of the newer versions anyway) - as it can be used as a random pitch generator as well as the many other things it can be used for

keep the es9 - as it will be your audio interface - and install either cv tools, silent way or vcv rack (vst version) to allow you to sequence from your daw... whichever works for you

get rid of the midi->cv module it's superfluous

and get a matrix mixer...

tiptop mantis is an excellent choice of starter case - I have one & if I were to buy another case (most of mine are DIY) I wouldn't hesitate to buy another mantis

also read my signature: and think long and hard about what it says... apply this to your thinking before you start spending money on modules!!! you don't have to follow the advice therein, but at least think about it...

"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia

Utility modules are the dull polish that makes the shiny modules actually shine!!!

sound sources < sound modifiers < modulation sources < utilities