This isn't a question about how to raise purity; the mechanics there are simply enough. This is more how do you do it without constantly shooting yourself in the foot?
First off, cabbage. What do you do with the seed drying time? If you plant your second best seeds while waiting for a new batch to dry, then you have ~1.5 days or more that your best seeds are sitting around. But if you don't, your fields lay fallow for a day or more. I've started sneaking in some massively coaled cereal into the cabbage fields during the drying time, but that's because my second best cabbage seeds suck. Plenty doesn't increase seed stalks as far as I can tell, so I just coal the crap out of cabbage.
Second, pumpkins. Unless you get lucky on a massive S&C, your T1s won't get nearly enough seeds to replant more than 2 fields. I note that I tend to get purity increases or break-evens around 2/7 or so, so that's not enough to sustain me without falling back on older, less pure seeds. Do you use some (read: lots of) lime? Do you wait for T2? What's the distribution of pumpkin stage growing times, because it doesn't seem to be 3/3/3? And how does speed fertilizer interact with it? I have observed normalcy, but other posts suggest apocryphally that it only works up to T1.
Thirdly, general considerations. How many fields do you dedicate to purifying one element/crop pairing? And what do you do with all the excess low purity produce? I don't play enough that I can cook it all before even more is done and it just keeps piling up and up. Maybe that's a sign I should make more ovens/baking tables.