I think to realistically get that range you need a battery that’s a bit bigger, riding and setup makes a huge difference for efficiency so it’s hard to tell. You might find it’s quite a long ride on an esk8 and you would stop and charge anyway.
Using 20S motor controllers on 14S is a little bit inefficient, on 14S 2800 watts they make about as much heat as they would with 20S 4000 watts. Could you switch to 12S11P which would let you use 12S VESCs like a DV4S? It would be much more reliable and compact than Flipsky.
I second using more powerful cells, you often get more energy overall from a strong high current 4.5Ah cell than a weak 5Ah one that has more voltage sag.
You probably know already but you will need to have a spot welder and be able to solder.
The battery builders club here on the forum is great for best practices and how exactly to do things.
Just connect VESC power leads onto the battery and set braking current based on what you want riding, with that size battery you won’t need more braking current than it can easily take.
Do you have a strong understanding of the difference between motor current and battery current settings? It’s really important and newcomers often don’t understand it