Uninstalling Mods

Depending on the mod, as well as how much you've engaged with it, uninstalling it may be easy or difficult.

Make sure to keep Error Handler installed. As its name suggests, it handles a number of errors.
Any items that are new (i.e. not an existing game item) like new fish, new minerals, new crops, etc. should be trashed/cut down/removed from fish ponds before uninstalling the relevant mods. This goes for items added via C#, via Json Assets, via Dynamic Game Assets, etc. (Note: DGA actually handles most of these fine but it's still best practice.)
Start by removing content packs, but leave frameworks in place. Typically framework mods don't do anything without content packs installed, but they can help catch various errors that might happen if you mess up and leave something in by accident.
If you are married to any mod-added NPCs, divorce them first. If you're cash-strapped, debug divorce exists.
If anything looks weird in terms of bushes/weeds/trees after uninstalling, you can use Reset Terrain Features to fix it.
If anything goes wrong (spewing red errors), quit the game so it doesn't save, then upload your log to smapi.io/log and read through it or ask for help to determine what you missed.
If you are uninstalling a map editing mod, you may want to keep NoClip around for retrieving any items lost to the void.
If you are uninstalling a mod that adds new locations, make sure nothing important is left there, because they will get erased.

Uninstalling any NPC mods requires you to play 1 day and sleep before the NPC is purged from the game. SMAPI will also show warnings in the log and a notification when you wake up, this is okay and normal if you are purposefully trying to remove NPCs.
Content Patcher mods that only change the appearance of things (so not adding new maps, NPCs, etc) are totally fine to remove/put back anytime.
Uninstalling SVE is slightly trickier only because it's a very big mod that you may have engaged with in various ways. Pay particular attention to the advice on items and spouses and you should be fine.
If you have fish pond errors, try installing Warp Snitch. Yes, I know the name has nothing to do with fish ponds but apparently it helps.
Uninstalling Json Assets content packs may give you a Json Shuffle, the same way that installing new ones can cause it. Keep Lookup Anything, CJB Cheats, and CJB Item Spawner around and you'll be able to fix anything you break that way.

If you would like to stop playing modded Stardew altogether on a specific save, you would start by uninstalling the specific mods you have, as described above, and then uninstall SMAPI.
To uninstall SMAPI, simply run the installer and pick the uninstall option instead of the install option.
If you want to switch back and forth between modded and not, you may want to look into mod groups.

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