What do I need ??

I think you need to study a bit some sound engineering basics, it will give you a precise overview in the craft and how to work with hardware devices

a hardware production studio (at least in my opinion and experience, others will have a different view) revolves around the
mixing console, which can be as small or big as your needs and wallet Considering inline consoles to be the standard,
you would have to have a mixing console with 8 to N channels, a number of auxiliary sends, an equivalent number of
return channels, from 2 to N group channels and the master volume.

Instead of doing the "cable jockey", I strongly advise you to buy a patch bay, which is usually a rack mounted panel with
a number (usually 24 or 48, depends on the brand/make) of 6.4" connectors (jacks) in the back and in the front.
You would connect permanently all the inputs and outputs of your devices (synths, the mixer, the effects, the audio
interface of your comnputer) on the back of the patch panel, then with short cables (patch cables) you would interconnect
the devices in front of the panel (like in the old telephone pabx system, that is where they come from

For the effects, you would connect one (or more) of the aux sends of the mixer to the input(s) of your effect, then the
output(s) of the effect would either be connected to the return channel on the mixer or to the normal channel (this is the
way I do, so that I can have a different equalization on the effected signal compared to the original).

Clearly, using a patch bay makes these connections way easier and practical than having to move cables all around

Your soundcard has 2 inputs and two outputs, this means that you can record two channels at the same time - either two
mono channels (two different instruments) or a stereo channel (one instrument with stereo output). If you want to
record more channels simultaneously then you have to get another audio interface, with more than 2 channels

I have an old Echo Layla3G which has 8 inputs and 8 outputs, so I could record 8 channels at the same time and play 8
separate channels in realtime - however I can already mix in the computer, so I can pass through more than 8 different
"instruments" at the same time, unless I need them separate to do some analog filtering through my mixer.

As for digital consoles, I am an analog guy so I never had one and can't really tell how they work.

Maybe I have confused you even more with my post, I hope other people will be able to address your questions with
more precision than I did

a.