The more I look at this design the more I feel it fits quite well within TOS. It has a more utilitarian look to it while stil fitting within TOS' Starfleet design language. By chance or by intent the TAS illustrator did quite well with this. I think the Huron freighter works as well.
The trick, I think, is to make it look somewhat less cartoony (in how it was drawn) and adding the right detail and colours to fit within the TOS aesthetic while not copying TOS designs verbatim. For example the warp nacelles. The nacelles of the Class F shuttlecraft are similar in pronciple to the Enterprise, but they differ distinctly in detail. Other Starfleet and Federation space warp nacelles should also differ otherwise it just looks like a direct kitbash.
The trick, I think, is to make it look somewhat less cartoony (in how it was drawn) and adding the right detail and colours to fit within the TOS aesthetic while not copying TOS designs verbatim. For example the warp nacelles. The nacelles of the Class F shuttlecraft are similar in pronciple to the Enterprise, but they differ distinctly in detail. Other Starfleet and Federation space warp nacelles should also differ otherwise it just looks like a direct kitbash.