Tail lights are easy, because they require a lot less power than headlights. Here is my approach.

More is better, right? I have a dynamo powered tail light. Because it requires little power, the standlight keeps the light on for longer than the headlight's standlight. And it keeps the tail light on at full power, whereas the headlight goes to low power as soon as I stop (which is OK).

I also have a battery-powered flashing tail light. I can easily remember to charge it more often than necessary, because I probably only have to charge it once a month. I think I do it twice a month.

The combination of the two tail lights is better than having just one.

I also use a spoke light, because it's a great attention grabber, even at low intensity.

If all of that sounds too complicated, then keep using your battery powered flasher(s) and don't get a dynamo powered tail light. You get the dynamo for the tail light. Finding you don't have as much charge for your headlight as you thought is a major hassle. It doesn't happen to me with my tail light. My dynamo headlight has not failed me once in several years of use. Not once. My tail light has failed two or three times, and the cause was always a wire that had fallen out, and riding without a tail light isn't as scary to me.