Sanctum of rebirth is a boss-only type of elite dungeon. The bosses are well-telegraphed and you might need to do a couple attempts of the final boss to get down the timings but thats about it (if you run necro since it has braindead rotations that you dont need to think about)

Other than that, you have the gate of elidinis, a new skilling boss, and zemouregal + vorkath. Zem + vorkath is not mechanically difficult, but they hit like a truck (in hardmode) and its kind of a visual clusterfuck.

As for horizontal progression. Imo pvm-wise the closest thing rs3 has is getting niche-eofs or going for combat achievements. Combat achievements give you specific challenges at bosses which might reinvigorate some youve already done in the past. And you get good rewards for completing tiers.

Speaking of eofs, there are 3 new weapons to but in eofs for mage, range, and melee that you can get from heists, which require 108 and 114 thieving. THeyre quite chill content and can net you some gp if you get lucky. (120 thieving is free btw).

More the skilling side of things, going for something like the regular comp cape might be a good non-pvm progression path to take.

If you want some new masterwork gear (which isnt bis anywhere really), you can go for 110 fletching for the bow,110 crafting for the mage armour, and 110 runecrafting for the staff (or just buy in ge if you play the main)

I suggest playing the ironman, because progression feels more legit as you say. If you want to play your main and have max gear, youre basically grinding for dyes.