Please note I haven't made a board in my life except some esp32 stuff to manage my home automation. But I'm good with soldering and have basic electronic knowledge.

 

So how do I go about my DIY route which will be an interesting learning route?

You can go through-hole for most of the parts. They are a little hard to find. I just go surface mount.
If you go surface mount, think about how to solder. For example, a R0201 resistor is like a flake of pepper, and I really try not to do that. The R0402 is also too small. I use R0603 and you might want to go to R0805. I like SM better, but it is a little at first. (just different)

When you get ICs, get TH or surface mount. On the SM there are several packages to choose from. DO NOT get the packages with the connections under the part. Get parts where you can see the leads.
This is the bottom view. You cannot get under this part to solder the pins. I can show you how but just don't do it.

Here is something you can hand solder. When getting these there are probably at least two sizes. Pick the one with the most distance pin to pin.

When making it yourself you can mix all different kinds of parts together. Be free.

 

Lots of helpful ideas around how to build electronics

 

So how do I go about my DIY route which will be an interesting learning route? Something I can either solder myself or order online. I'd start with a two channel board to order online so that I can get 5 boards for a total of 10 channels. Something that does not require tuning once created.

? Seriously . . .

 

I mean fixed values. Fixed speed/latency according to the mesa 7i77 board requirements.

Parameter	Your System's Requirement	HCPL-7840 (Simple)	IL300 (Complex)	Verdict
Required Bandwidth	> 500 Hz (5-10x servo rate)	100 kHz	200 kHz	Both exceed by 200-400x
Max Allowable Latency	< 100 µs (10% of servo period)	~10 µs	< 10 µs (circuit dependent)	Both meet easily
Required Slew Rate	~0.5 V/µs (for 10V step in <20µs)	~8 V/µs	Circuit Dependent	HCPL-7840 meets
Critical Need	Stability, noise immunity, simplicity	Good CMRR, integrated	Excellent linearity, discrete	Simplicity wins