I wonder if you need to change how it works normally, so that it doesn't need that focus.
OBS can be made to be a slideshow, as you say you're doing, but it's not really meant for that. It's more of a free-form TV studio. My first thought is that a slideshow should probably run from an actual slideshow app, like PowerPoint, LibreOffice Impress, OpenLP, ProPresenter, Keynote, etc... Lots of those to choose from, with a wide variety of functionality and price, that don't necessarily correlate with each other.
I like OpenLP myself. It's free, and has its own remote control interface, so it doesn't need focus at all. Just turn it on in the settings, download a phone app (NOT from the app store, but from the QR code in OpenLP's settings), connect the phone app to the desktop app, and you have a dedicated controller that doesn't lose focus.
If you're only running a local display and speakers, then you can run the show directly from OpenLP, without OBS involved at all. Or you can capture it in OBS as part of a larger production.
I expect other apps to follow a similar idea, with variations on how the remote works if at all, how you set it up, what you can do with it, etc.