The last thing would be to reduce the box size around each element so the glow emanates from the control, not the outer box.
There is no “outer box”. The control itself is the box and the “glow” is just the visual indicator of exactly this control. You can try to make the controls smaller – but this will also make them harder to click or touch.
It is not possible to create controls based on clickable <button>
elements which have the shape of a circle – this is only possible with image maps but this is a completely different concept to design the UI and would also require substantial changes in the UI of Photoswipe as it does not use images as controls but buttons with image as visual indicators.
However – feel free to create your own UI – see “How to modify the PhotoSwipe markup” in the FAQ. You can apply a filter for lbwps_markup
and provide your own HTML code for the Lightbox wich does not use buttons but images with an image map. The only important thing is to use the same CSS class names, so the Photoswipe click handlers will work properly.