Fruiting in pots can be considerably quicker than in ground. I've had several trees (not just saps) that were flowering/fruiting in a pot, and then after planting in ground, took their time to start flowering again. When the roots can grow freely, thats where much of the energy goes. I'm convinced that a confined root system is one of several stressors/triggers for flowering.
My guess is that a sap tree would produce well for how ever many years, then decline, if in a pot. Proper care could mitigate the decline. Having a new tree ready to take its place would be ideal imo.
Yes, many saps will flower when small. But they cannot support adequate fruit development, and will miss out on needed growth if putting energy into fruits. Wait until branches are numerous, and can support some weight.
Based upon the 3 I've grown (Alano, Silas Woods, Butterscotch), they all seem to be slow to moderate growers. Alano was the most coy in flowering. Silas Woods is a consistent producer, and is still about 9 - 10 feet tall. Beautiful tree. BS is flowering now too, and hopefully will hold a few fruit. Its only 6 feet tall and skinny, so would only let it hold 3 or less.
Your protection of 3-4 years is reasonable, in most situations.