I haven't played it for myself, but Pokemon Refined Platinum exists. It looks like it would be right up your alley with the difficulty. I doubt it's brutally unforgiving in terms of battles, but the hack looks like it could be worth a go.
But whatever you do, do NOT go anywhere near Renegade Platinum. I swear, it's probably going to make you hate Sinnoh in its entirety. There's difficulty, all right, at the expense of a TON of forced required battles that weren't in the original. There ARE some good things, like being able to catch all the Pokemon and some stat/type/ability changes for others, but all in all it goes a little too overboard with the things it wants to do. Unfortunately, it's also most likely the best Platinum ROM hack out there even though i despise it with every fiber of my being.
However, there are two add-ons to Platinum in general that i will absolutely recommend. One of them is Following Platinum, which adds the HGSS feature of your Pokemon following behind you in every area of the game. Sometimes your Pokemon will interact in cutscenes, like your starter mucking about in Rowan's lab after you visit Sandgem for the first time. Another mod is the individual color variation patch, which changes a Pokemon's color hue depending on its IVs. Both patches are worth checking out!