You could call them "small agricultural businesses", which seems quite common and thus avoid the problem.

A small agricultural business is defined as one having an Estimated Value of Agricultural Operations (EVAO) of between $22,500 and $400,000. Businesses

I'm not 100% sure about "collected" for "retenido" because I'm not sure whether it refers to collected VAT that the firms have already transferred to the government, or that they have collected it and have not yet transferred it to the government.

AG - for agricultural
SME - for "small and medium enterprise".

"SME" isn't perfect for "PPA" because it seems like it includes larger producers (the "medium-sized" ones).

However, I would separate between "smallholders" who may live very close to subsistence and probably produce very little for the market, and people who are truly running a business with accounting analysis, cost-benefit strategizing, etc. going on, and in this case I think it becomes appropriate to group together these "pequeños productores agricolas" as "agricultural small and medium enterprises" (AG SME). The problem is one of formal classification, because SMEs often receive specific tax treatment, so it may not be correct to group them together. For example, there is often a VAT threshold (in Canada, a small producer pays no VAT under $30,000 annual revenues).

Anyways, I propose this as a likely suggestion and highlight some issues which need to be considered to be sure about the correct answer.