I agree that luck improves the chance of seeing either a +1 or a +2 bonus but to even have a CHANCE of getting those bonuses you have to follow this rule:
ROTATION:
plant potato or grains after vegetables and vise versa
flax and grapes after free tile (nothing was planted before)
QUALITY:
If you use the right crop rotation sequence, then there is a chance to have +1 to the seeds quality;
While harvesting, there is a chance to get +1 to a harvest. You should notice: the quality of the harvest is determined by the lowest of four factors: Farmer Skill, Seed Quality, Soil Quality, and Water Quality.
NOTE:
Max bonus to quality is 2 points.
You don’t lose crop quality due to improper crop rotation
So based on that you need to have crop rotation AND skill, seed, soil, water at equal levels to receive the +2 bonus. I don't believe having an insane amount of luck is going to get you +3, +4 bonuses and up.
Are you saying that you planted 30 quality soil, 30 quality water, 30 quality farming skill and 30 quality seed. Then ignored the crop rotation. Pushed your luck as high as you could get it and are seeing +2 gains
specifically for flax?
If i read that correctly you said you went to 31 soil quality 31 seed quality. I'm assuming 31 farming and 31 water. THEN you reached 32 seed quality? That would mean you're still only getting +1 gains.