As
@Hadron commented on, restoring those deleted photos and videos isn't likely. So try not to get your hopes up there is some 'magic' fix, it's complicated and pricey to restore deleted data off of the digital storage media inside your S9 Plus.
Moving forward, it's really important you set up a backup solution so this doesn't happen again. Some people have become hyper-sensitive to relying upon online storage services so if you have a similar mindset, I'd recommend setting up SyncThing. It's an entirely free, Open Source project that allows you to sync your files locally (all within just your own home network) or to your own encrypted online storage server. It's more involved to setup and implement, but a reliable way to keep your data automatically backed up and away from prying eyes.
https://play.google.com/store/apps/details?id=com.nutomic.syncthingandroid&hl=en_US
https://en.wikipedia.org/wiki/Syncthing
https://syncthing.net/
If however you're willing to let the big corporations do their data-mining on you (a price you pay for free, automatic ease-of-use), give some serious thought into just switching away from the Gallery app and use the Google Photos app as your primary photo manager:
https://play.google.com/store/apps/details?id=com.google.android.apps.photos&hl=en_US
A helpful feature in Google Photos is an automatic backup and sync feature. It runs in the background and backs up all your photos (and videos if enable it in the app's settings menu) into your online Google account. Once implemented, you then have access to all your photos on either your phone or using any browser by going to
https://photos.google.com and logging into your account. There's also a feature, accessible in both the app and web interface, where deleted photos are restorable up to 60 days (unless you manually clear the Trash) so if you do inadvertently dump a bunch of photos again you have a lot of time to correct your mistake. If you have a real liking to the user interface in the Gallery app over the Google Photos app, you could just continue using Gallery as your default photo manager with Google Photos running in the background doing its backup & sync.
Also, install and setup Samsung's Smart Switch utility on a computer.
https://www.samsung.com/us/smart-switch/
Smart Switch is a handy tool to keep on hand when you have a Samsung device. It allows you do full backups that make doing a restore of your account a lot less of a hassle in the event something really bad happens.