The diagram you show is not coma as coma is a radial aberration, i.e. stars will appear comet like with their tails radiating away from the centre. Coma is linear and increases in size further away from centre. The aberration you have shown looks like sagittal astigmatism and more likely a combination of aberrations.
See
https://www.handprint.com/ASTRO/ae4.html (scroll down)
I do not know enough about aberrations to be able to comment further but I can tell you what you are seeing is not pure coma from the primary mirror for once you see it and it alone you may like me be surprised how small it is off axis and even call it beautiful as I did the first time I saw it and it alone.
Ask at Cloudy Nights too and show your diagram if you don’t get a satisfactory answer here. Note not every answer will be entirely factual because unfortunately people tend to copy what others say even if what others say is flawed and without having any experience with the problem at hand. The urban myth about needing a coma corrector below f5 is just that an urban myth. You don’t magically need a coma corrector below f5 and not need one above f5. It is a personal preference and if you find coma objectionable you may want but not necessarily need to use a corrector at higher f ratios or choose not to use one at lower f ratios.
I think first you need to determine the cause of the aberration you are seeing before investing in a coma corrector which is design to correct for primary mirror coma only. Perhaps others can help you determine this cause.