Need help adapting running balanced encounters for level 10 and higher

I am running essentially a sequel campaign to my Curse Of Strahd campaign I just finished not too long ago. The starting level ofc is level 10 and the campaign we are running now is Vecna. I am already in the process of running one shots for all the players to get fimilar with my world. I've always been more roleplay focused and only doing combat encounters when necessary to plot but my players request a bit more combat for Vecna. I wanted to combined elements from other modules (for example Tomb Of Annihilation and Avernus) to it but I've noticed really quickly most of those modules go up to level 10 or even lower. I want to have more combat but also make it balanced and engaging for the players. I know the basic stuff like I can't rely on smaller creatures like kobolds and goblins and I need to add more challenging enemies.

If it will help here's a quick summary of my world: it is a large continent with a couple islands around the place. Pretty much everything you can imagine is in this place. From the mountains of Briar Blia which is the main place where dwarfs live and a fairy forest of Mystonia. In the middle is a place called the Red Sand Valley where Vecna's old kingdom once stood before Kas fought him in the War Of The Gods where Vecna's first attempt of world domination happened. It's said that the reason why the sand is red is because it was stained by the blood of those who died in the war. This world is on the verge of a steampunk industral revolution with new technologies being made off the coast on an island called Dredge. The technology hasn't gone mainstream just yet but give it a few more years and it will happen.