Chip load / thickness is for the most part only available from reputable mfgrs but one can often use similar values for generic brands / unspecified end mills.
I use this one: http://www.sorotec.de/webshop/Datenblaetter/fraeser/schnittwerte_en.pdf
V-bits as you have experienced dont provide a good “floor” finish. V-bits can also provide issues shwoing in your picture, a “dog-bone” corner of the letters. This is probably because the V-bit used have a minute flat spot causing the bit to go a minute step to deep.
A 1mm 1F end mill would be my go-to bit for plastic engraving provided the design isnt too small or intricate.
A 3-5mm flute length is all that is required.