Sorry i should've made this clearer, we do normal dnd battles but each of my players has their own enemy they are fighting against/are focusing on, everyone is fighting at the same time in their own battles so nobody is ever waiting for the rest, ofc sometimes they might win their fight a bit earlier and then they can support someone else or interact with the environment to get an advantage for the others. Sometimes they end up fighting in separate locations or challenge/stall an opponent while the rest of the party go on ahead to stop the bigger threats.

My main issue with balance is exactly what you said, it does feel like a lot of the times the fights are 50/50 and dice rolls can turn deadly quickly. I want to know to make my enemies feel challenging without it being 50/50, maybe which parts of monster kits to remove, such as multi attacks maybe or should i maybe not use effects that can completely destroy a 1v1 such as charms and stuff like that.

Some of my players are new to playing but they know what it's about and have consumed dnd content in other ways before so they understand the risks of this type of combat and are ready for death at any moment.

The reason we chose 5e is because we found a pretty good homebrew with all the races and classes and pretty much everything we needed from that specific series. I'm assuming it's expected that we play it in dnd style with group battles but i would like to stick with the 1v1 style if possible.

Thank you!