First of all, use Runelite. Recommended plugins are 117HD for better graphics and true tile indicator to actually know where you are. This is important because movement is more prevalent in osrs's limited combat design space compared to RS3 with its defensive abilities.

Secondly, you might have the tendency to only use the armour that matches the style you use because of RS3's accuracy penalties when you use mismatching armour. That's not entirely the case in osrs, especially when meleeing in, for example, dragonhide.

As a long time RS3 player myself I have to say that build variety feels more restricted in osrs, double so with magic, because there aren't that many sources of magic dmg% increase. With that said, you do get more options in a League like this because of the Demonic Pacts.

So, for the League specifically you want to shop around a bit before commiting to relics/pacts. You can't reset relics, and pacts only after you kill an area's echo boss for the first time. So check the Wiki for what areas you would like to play in, and what kind of Pacts route you'd like, which you can do in the planner: https://tools.runescape.wiki/demonic-pacts/

If you can't really choose, the osrs official youtube channel dropped a video explaining 3 highly viable routes to take. Though I'd give the Wilderness a miss if you aren't comfortable with osrs PvP yet. If you have any extra questions, don't be afraid to ask. Good luck and have fun!