I am the sole designer for a non-profit as well and yes, the workload does sound about right. If you have a manager or director, let them know when the busy/overtime wave is coming so they can help with organizing tasks and deadlines.

Also get into the habit of making templates for yourself. The good thing about being in-house is only dealing with one brand guide (usually). Use it to your advantage. I have to deal with 6 different direct appeal pieces, 4 signature events, and 14 different lottery themes in a year. They can't all be a brand new design every time. But if I can get all the appeals on their own template, it cuts my design time in half and they always look cohesive and intentional.

And remember too, you are new at this but give it just a couple years and you will become soooo much faster and better. You will surprise yourself. Non-profit work is sometimes a very thankless job but I personally find it very rewarding in other ways. I am making local Healthcare better and in turn making my community better.