You don't say whether your phone is a North American model (qualcomm processor) or a Vast Majority of the World model (exynos processor). This makes a huge difference to how you root it or even whether you can, and what ROMs or mods are available for it once you have.
These different models may have the same marketing name, s8+, but are different phones internally, and mods intended for one will not work on the other, so it's very important to use the right software for your phone.
And yes, you can save the original ROM if you can unlock the phone and install a custom recovery without modifying it. But that's not enough to return it fully to stock software, for that your best bet is just to reflash it with a full set of official software. Even then it will be clear that the phone had been modified, since rooting will trip the Knox "E-fuse", and software that checks the Knox status will detect your phone as having been altered.
Also if you use any apps that don't like running on rooted phones (payment apps, banking apps, some media apps) make sure that whatever root method you use does allow you to hide the fact that it is rooted from the specific apps you care about.
In short, do your research before you try to change anything.
