4,000 trees? Wow!
At that volume maybe you could seek out a partner with processing capability. You need market differentiation as the local market is flooded, mostly with one kind of mango.. Organic or place of origin are possible differentiation. Organic gives you a possible way to command a higher price, especially for export. Switching to organic would likely depress yields until you can revive the soil and learn what organic compatible products will help take care of your orchard. Those sprays may seem pricey but you only need a little product per tank and the commercial products keep getting more expensive. You might ask Dr. Treelove, as he took care of and/or owned a commercial organic mango grove in Thailand. Best of luck.