I’d say keep one axis locked, expand/contract the other, based on whether the aspect ratio is wide or tall, and based on a specified size/apothem of the camera.
Unity does this partially. It can only lock the Y axis. (Although you can alter it with code to lock on X if you like)
If you set it to orthographic, you can then specify the size/apothem of the camera on the Y axis, the X axis size will then auto-adjust based on aspect ratio, and the size/resolution of the screen doesn’t matter, as the camera will still show the same content (As in, 1920x1080 will show exactly the same amount of content as 1280x720, since they’re both 16:9)