Rules for barrel storage.
1. Exposure to the oxygen in the air will cause wine to spoil. Therefore, do not put wine leaking from the tap back in the barrel, drink it or use it for cooking.
2. Wine barrels loose moisture to the environment. You should check the level of the wine in the barrel every three months under normal circumstances. But, with a leaking tap you will need to check much more frequently. If the barrel is airtight, the loss of moisture will cause the barrel to pull a vacuum which you will notice when you remove the bung. With a leaking tap, your barrel is not airtight, you should begin by replacing the tap. You should be able to get a replacement tap at a winery supply business. You can replace the the tap by turning the barrel on end. If no tap is available, I would still remove it and replace it with with a wooden or plastic plug. Once the barrel is airtight, you should be able to age wine in it.
3. You said that your 2019 vintage was a little light bodied. Barrel aging will correct that problem. Loosing moisture concentrates the wine flavors and topping the barrel with wine will enhance it.
4. When the wine has aged to your satisfaction, empty the barrel, bottle the wine that came out of it, wash out the barrel and refill with more wine.
5. If the barrel is going to be unused for a period of time, it needs to be sulfured. This can be done with a sulfur candle or with a solution of potassium metabisulfite. You can get the metabisulfite at a winery supply store.
6. All of your homemade wine, whether in the barrel or carboys, and at the time you bottle it should be treated with metabisulfite to concentration of no more than 25 parts per million. Metabisulfite is the preservative that keeps wine from spoiling.
7. Every time you remove the bung to check the wine level you need to sanitize it by spraying with a solution of 20% metabisulfite or alternatively you can use unflavored vodka, neutral grape spirits (grape vodka). In a pinch, use grappa or Evercleafr ( 190 proof grain alcohol) if available in France.
7. Absent a wine lab, you have to rely on your nose and palate to determine wether the wine is ok or has begun to spoil.
8. Alcohol can be determined with a hydrometer which is satisfactory for home made wine. If you were a winery, selling your product, you have a lab determine the alcohol content. In the US, the amount of excise tax a winery is required to pay is based on the volume produced and the alcohol content.