Mk2 too big for a rocket?
Is this by design?

Makes a chore to deliver all those items only to build it in orbit rather than just launch the cannon to the platform with 1 rocket.

yes this is by design, what would be the point of an extension if there were no challenges?
if it's too tedious for you then don't build any
There was no Mk2 until recently, so you can also play without it,

Except that the last research locks you out of building Mk1 (or so it says, I havent researched it yet thankfully).

I would like to request to be able to still build Mk1 after the tech is researched, so I can upgrade my exisintg cannons while still being able to send less powerful ones in space without having to go through all the process of sending one million rockets full of parts (i hate sending rockets, the animation is sooo slow).

Except that the last research locks you out of building Mk1 (or so it says, I havent researched it yet thankfully).

This is the only reason why there is an extra research for the upgrade ;-)

I would like to request to be able to still build Mk1 after the tech is researched,

You can do exactly that. The purpose of the upgrade research is to finally switch to Mk2 ^

Parallel use of Mk1 and Mk2 is currently neither technically possible nor planned.