Suggestion for speed/ optimization?

Sorry, I was on vacation the past week, without access to a computer.

So caching plugins don’t work unless you exclude pages

This is only true for Google Photos, and that too, when you are using the PHP load mode. There is no issue with using a caching plugin for SmugMug or anything else.

most page optimization plugins with lazy loading sometimes brake the galleries.

That is true. Various layouts require predetermination of image sizes, and where possible, Photonic supplies the image sizes via the width and height attributes in HTML. However, most lazy loading plugins replace all images with generic, square placeholder images, which break layouts.

If you are using the JS load mode, you really shouldn’t be getting bad page speed scores. The only thing that the JS mode can adversely impact is CLS (Cumulative Layout Shift), and that happens if your gallery is at the top of your screen and not below the fold.

Dont all slideshow templates use PHP code even for SmugMug ones? making it so I can’t cache the page unless excluded?

Practically all of WP uses PHP for the backend. PHP is just the language that the server uses to generate the HTML markup for you. Once the HTML markup is generated, that markup can be cached by a caching plugin.

The only scenario where you wouldn’t cache something is if the content is dynamic. E.g. if you are pulling photos from a SmugMug album, and you are going to add more photos to the same album, then caching the markup makes no sense. But if it is a fixed album with no new photos, there are no issues caching it.