The act of going from a 3x to a 1x setup is not what will help with the chain slap. Upgrading to any derailleur with a clutch is. The 1x chainring is almost always smaller than the middle ring of a 3x(where most people spend the majority of their time), so that in itself can make the chainslap worse. The solution is just making sure you’re in a big/big gear combo when on the bouncy stuff so it will pull the chain sufficiently tight. Instead of small ring up front and 5 gears in on the rear, choose the middle ring up front and towards the granny on the rear. It will give you the same leg RPM but is a much better combo for reducing chain slap. If you never use the large ring, take it off and remove enough links in your chain to cover second ring/granny and you’ll improve your situation a bunch. Not to talk you out of going 1x but if that’s the primary reason you’re doing it, I’d save my money.
As for your fork, going from a 110 to a 140 is a huge jump and is going to make quite a difference in how the bike handles. 30 mm will slacken the steering making it more stable at high speed and will make it harder to steer in the tight stuff(because it wants to keep going straight). It will also affect your climbing on steeps as it’s tilting you off the back of the bike more. If you’re set on this type of change, I’d look for a fork that allows you to select between fork lengths. I personally wouldn’t want to be married to that type of change 100% of the time.
Dropper posts are always a good idea 🙂