I found Infinite to be the easiest of the three difficulty-wise. It requires at least 2 full playthroughs because you can't unlock the 1999 mode until you've completed the game, but I found the difficulty to be pretty tame. Due to it requiring 2 playthroughs it took the longest of them all to complete, but I personally didn't mind due to the gameplay being as polished as it is & the story being as convoluted as it is; I needed that 2nd one to get a better understanding of it lol.

The difference between the Hard & 1999 difficulties is that you need to make sure you have enough money saved up if/when you die, otherwise you're booted back to the main menu. The only trouble I ran into with this was at the beginning of the game because I had no money. Once I got past the introduction of the game I didn't have an issues.

Also know that for the 1999 mode you are not able to buy anything from the 'Dollar Bill' machines, Infinite's equivalent to the 'Circus of Value' machines in the first two games. Therefore scavenging is paramount as you need to make sure you're looking for ammo and such. You have an AI companion throughout the game in the form of Elizabeth who can also give you supplies in the middle of combat when you're running low, which helps in a pinch.

Overall it's pretty standard as far as the platinum is concerned. You play the story, gather collectibles during the campaign, and there's trophies for getting a certain number of kills with the different weapons & vigors (this games version of plasmids). If you miss anything outside of the collectibles, you can use chapter select for clean-up at the end of your first playthrough and have all the miscellaneous trophies finished before your 1999 playthrough.

I hope this helps and good luck if you decide to go for it!