Besides direct subsidies government grants, equipment, indirect subsidies can be in the form of incentive like tax rebates, low-interest rates, R&D incentive, etc. The efficiency of subsidies is subject to debate. I.e. in a corrupted country, subsidies rarely reach the needed hands.
Are there any major stable countries (population over 5 million) where there are no forms of agricultural subsidies of any kind?
According to the definition of subsidy, the short answer is: No.
BareBear in mind that: Advanced of agriculture technology for food security is way beyond the limit of "free market". Since the free market is all about profits, there is always unfair game such as monopoly, sanction, etc.
In addition, the agriculture business is highly susceptible to climates and diseases. An outbreak; drought; etc can destroy the agriculture worker income for the whole season.
I see there is an answer on New Zealand dairy industry, which needs scrutinize on details, e.g. "rainy days funds", incentives, low-interest loans, etc.