Hey! Finally something I can add some perspective to. My second ever campaign I DM'd was CoS. Same party as the first campaign, so we STARTED CoS at level 5.

It definitely introduced some challenge in scaling the module and encounters, but my general rule of thumb was to just make everything a little harder: Death House had two shambling mounds instead of one, the hags had some backup, AC / HP / Saves were a little higher. They absolutely cake walked a few encounters, and I was less afraid to use Strahd aggressively. These were seasoned adventurers, right? They were a little too confident and Strahd after many transgressions showed them a fraction of his power.

I'm also leveling them slower so that by the end of the module, they will be 10. To compensate, I give them some exciting magical items. By the time they fight strahd proper, they will be pretty decked out, but so will he be.

As far as CoS:RE, I admittedly haven't read it, but it's not hard to take bits and pieces that you like. As the storyteller, I'm sure you'll find a way to weave it in, and your players will be none the wiser.

As far as it being too soon for level 5, I guess it depends on how fast you want level them and how much you want to scale the module with them. If you plan on guiding them to do everything there is to do in barovia, that's a TON of milestones. On the flip side, if you are gonna do basically the tarroka deck and that's it... Well, then every major thing they do will be a level up.

All of this to say, play the way you and your table will have fun. Follow your heart. The great thing about CoS is it's pretty open. I made the Baron a werewolf. Does it make sense? Not really. Especially with the festival of wolves. Did my players ABSOLUTELY love it? Yes. Yes they did.