Second answer first; I thought of this one after the one below, but it's a better suggestion.
Use a power steering pump from a scrap yard. The pulley and belt system will allow you to power it from your axle, built with a suitable tensioning system.
Join the feed hose to the return hose, with a needle valve between. The amount of drag will be easily adjustable. If there's too much drag or not enough, a different needle valve assembly is an easy change.
This method is less complex than my earlier suggestion, less bulky, less experimentation required and likely to be as durable. Automobile power steering pumps can last the lifetime of the vehicle and are subject to pretty severe conditions. Some risk exists when sourcing from a scrap yard, of course.
Earlier answer follows:
One method of providing resistance force is to use a fluid that can be impeded while in operation. There are exercise devices which have a rather large fan to provide resistance/braking in such a manner that as effort increases, resistance increases, similar to real-world operation of a bicycle. Generally, aerodynamic drag increases by a factor of the square of the increase. A doubling of speed will result in a four-fold increase in drag. This is likely not to be a factor in your objective.
For a smaller profile of similar construction, liquid is a fluid that can provide similar results. Motor vehicle automatic transmissions use a torque converter. In simple terms, it's a pair of fans which push liquid (transmission fluid) from the power side to the driven side.
For your application, you could have an axle passing into a sealed container containing a fan-shaped impeller device. If the axle is vertical, perfect sealing is not required; let gravity keep the liquid in the cylinder.
Some experimentation would be required to determine the optimum shape of the fan. One could vary the level of liquid in the tank/cylinder to adjust the level of drag. Additionally static vanes on the perimeter of the fan (additional, perhaps unwanted complexity) would create turbulence and increased drag.
Additional layers or levels of fans on the axle would also provide for drag increase.
The highest wear points on this type of construction would be the bearings. If a lubricating fluid is used (automatic transmission fluid), wear would be minimal, as the bottom bearing could be completely inside the tank and the top bearing would experience minimal forces for wear.
To translate from a horizontal axle to a vertical one as described, the right angle gearing could also be enclosed in the drag cylinder, although many right angle mechanisms are designed to be used in extreme conditions and are going to have robust components.
