Minoxidil is safe for the beard area
First of all your testosterone is normal. There are many hormone replacement clinics out there that encourage men to keep their levels above 1000 but then you are at risk of losing your scalp hair and developing other problems. Taking testosterone will not make your beard thicker. Minoxidil can increase growth of your beard hair but if you see improvement, you will have to continue using it in order to keep that new growth. Once you stop using minoxidil, the hair would go back to "normal." Using is long term is perfectly safe for many years. Occasionally patients develop allergic contact dermatitis to the other ingredients of minoxidil so if you develop an itchy rash you should stop using it. The only permanent solution to a thicker beard is hair transplantation. You can take hair from the scalp and transplant it into the beard and the results are very naturally and long lasting. We do this procedure at my clinic often. You may want to look into that procedure as a permanent solution but you can always try the minoxidil first.
If minoxidil has worked for you and you were able to grow hair, you will need to continue using minoxidil indefinitely. If you stop using the drug, the hairs that grew will fall out.
Beard hair or facial hair is an inherited trait. It is genetics.