In PSE 2020 Captions in "Full Screen" mode are in the middle of the picture. I want the captions at the bottom of the screen. I would upgrade to get this.
Same question here:
The 'Full screen' mode was not intended to work as a real slideshow player, rather to offer an alternative for viewing files rapidly and in full resolution for selecting and culling import batches. That's many versions since this mode has got new features. And the slideshow features in Elements have changed to offer only automatic slideshow creations without the older customization tools like slide duration, transition effects etc. The choice from Adobe is now to use the Premiere Elements companion to create really advanced slideshows and videos.
So, why not look for alternative free software solutions for more customization? There is no harm to use also non Adobe solutions for specific needs not fully covered by Elements. In your case, I would use Faststone, Irfanview or other tools provided by Microsoft or Apple.