Apache has the better open source tooling IMO.

I use both, but at work I prefer apache simply for its relative ease of setting up our SSO solution. There is probably a tool for that in nginx as well, but its either proprietary or hard to find (and I did try to find it, but setting up and learning apache and then SSO was actually easier for me).

That’s great actually.

Then I would suggest using used thin clients. They cost around 50€ each, maybe less. You can install a Linux on there and remotely manage them then. They are quiet, small, not energy intensive and mostly have an x86_64 CPU, so software is also not an issue.

They also mostly have HDMI so connecting is not an issue either. If you still buy a Bluetooth remote, they can be handled without a mouse.

Software wise I am not well versed with google slides, but you can probably use kiosk mode in Firefox or chrome and just have the main page with the slides as chosen website.

Okay, let me get your current setup/needs right:

You have multiple rooms with dedicated Raspberry Pis, that each run PiSignage to display automatically forwarding (google) slideshows.

You now want to minimally change this setup to allow people to manually forward slides.

This begs some questions:

1. How are the slides selected/uploaded?
2. How are the slides forwarded?
3. How does the setup know if it’s an automated slide or not?

In my head the new setup would look something like this:

The Pis stay, as does PiSignage.
A device is added to forward slides (most likely a Bluetooth remote)

Here is where it gets tricky.

On remote press, a menu could be opened, to select uploaded slides and display them via other means than PiSignage. Closing this slide opens PiSignage again.

But having this easily maintainable is tricky and it will get hacky and people will forget closing their slide and so on.

Alternatively USB-sticks could be used.
Inserting one opens the folder, a slide can be selected with the remote and removing the stick opens PiSignage.

Both methods are hacky and not easily maintainable. But I can not think of other means.

Also I think that you should first think about some means of uploading and selecting slides as well as whether you even want to keep using PiSignage.