Can you still keep the hair you’ve gained from minoxidil if you generally lower the dosage?

If you stop using minoxidil, even weaning yourself off gradually, you will lose all of the effects and t will be like you never used it at all. 

While minoxidil works for some, the problem with the medicine is that it does not stop hair loss, and the DHT will continue is binding to the hair shaft which will eventually overtake any additional growth caused by the Minoxidil and die, perhaps just slightly later than it would have without the medicine.
Also, the alcohol base required to support the active ingredients also cause irritation of the scalp and severe reactions for many people, such as the increased shedding you are describing. The fact is that it does not work for everyone.
The best thing to do would be to see a doctor who specializes in hair regrowth and the rebalancing of body biochemistry who can diagnose the cause of your hair loss. 
Any medicine that actually can make changes inside your body, are required under the FDA to be available by prescription only. This therefore means that it is not possible for anything that does not require a prescription to cure or prevent hair loss or regrow thinning hair. Thus, a personalized medicine will be the best choice because instead of a "one size fits all" approach, you will be taking something which is truly catered to your body's needs.
The treatment is out there. And you can still save your hair! Good luck on your search.