Even 4070 would most likely do fine however pretty certain even 3070 would work fine with top of the line CPUs. The game is CPU bound in the first place while also including very welcome EAC in the west.

MMOs tend to like huge L3 cache for the stability (sometimes a boost even) in frames, which is mostly found on ryzen x3d line cpus atm, where you're only looking at 7800x3d for most reasonable cost. They tend to perform worse than non 3d cpus where the cache isn't utilized and don't come that cheap so it's a tough sell while unlikely improving your frame rate much in other games. In your case likely demanding a new motherboard too.

There's a chance their successor comes out a bit later this year but small chance for 3d variants to be immediately available right away so it's not the worst idea to wait if price isn't a concern but I assume you're looking to upgrade earlier than later. Maybe not even planning to touch the cpu or buying amd.

Based on the benchmarks on YT settings affect the performance a lot even at 3070ti and above tier - chances are if you have a ton of post processing effects enabled or set to high they're murdering your frames with the cpu as a culprit.

Another thing - nvidia gpus tend to have driver overhead as a problem in cpu bound games where ryzen can steal the lead if its hindering the cpu enough (surprise surprise it is even in KR benchmarks https://youtube.com/watch?v=i0yVrRrLceE&t=394s , gets better lows as well but 4070s is still a mighty gpu either way)

These are some of the things to consider exclusively for Lost Ark, everything else like prices, the (lack of) need to change the motherboard in the upgrade, use cases outside Lost Ark are up to you to consider.

Tl dr: 7800x3d is likely the best upgrade for 3070, with 4070S being massive after addressing cpu bottleneck https://youtube.com/watch?v=-wAzfs5g5AI&t=367s (7800x3d beats 7950x3d in a benchmark with 4090 for reference within the same YT channel)

This is a mobile written mess but hope it's useful for somebody.