None of us have a plan B. Finasteride and dutasteride are the only clinically proven treatments to work in the long term. There's things in the pipeline like pyrilutamide, but this goes back to the old meme "another treatment will be out in a few years". Some of the treaments in development by Kintor are promising, but I wouldn't bet on them.
If you get side effects from finasteride then it's going to be up to you to weigh the pros and cons of if they're worth dealing with. Some people continue with side effects, some don't. If I had the side effects that
@Stating facts had, for example, I'd quit the medication as well. That would not be worth keeping my hair over.
With minoxidil, the most you can hope for is probably somewhere around 3-5 years, but it also depends on how aggressive your hair loss is.
I think the only thing you can do in your case, if dermatologists can't tell the difference between vitamin D thinning and DPA, is to monitor your vitamin D levels and the quality of your hair. If it continues to decline, then you know it's not related to a vitamin D deficiency and you should follow up with a dermatologist about finasteride. If you worry about this day in and day out, you're going to drive yourself insane.