Kind of depends on how you play them I guess. If you do them purist style, then DSR and DS3 are roughly the same? Each has some bosses that are hard, DLC is harder than the base game. Hard to rank. The DS games are made easier by using the NPC summons or players. I personally like that by doing "side quests" and talking to certain characters add their npc summons to bosses.
Nioh games are much more action / arcade oriented than the souls games, but can be very difficult due to far more aggressive mobs and more of them. Unlike Souls games though, you don't just scale a weapon for more damage and level some stats, there is a whole skill tree for each weapon, magic and ninjitsu. You can easily look up OP builds that will make the games significantly easier. Same with weapons, some do much more damage more easily than others. IE: Nioh 1 - axe and Nioh 2 - odachi. It can also be made easier by summons since other players (even higher level ones) can add their summons. For instance you can find level 50 people to summon in front of level 20 bosses (not as OP as it sounds as your equal level summons are basically useless and die after one hit from a boss). You can also battle other peoples graves for gear. Some magic skills (like slow) also has a large impact on some mobs that are very hard, but slowed become very easy. You also have skills that respawn you upon death.
Code Vein is easier all around than the rest of the list. It's also easier to grind up to progress through power rather than skill.
Sekiro is probably the "hardest"? Since there is little point in grinding for souls to level up and no summons. Upgrades are won by minibosses and fights. You either learn the parry system or you don't progress. For someone who probably blocked / parried less in all the DS games combined than the first area of Sekiro, it just didn't click with me. I've never got very far in the game (past snake and guy on horse). Got to the temple with the screaming guys. I wanted to like it, but.... just didn't. It's a very one minded game so to speak. You have one sword, no upgrades to it that I know of. You get some gizmos to use, but in reality, every fight is won by the parry mechanic.
Not sure if you played them, but would also add The Surge 1 and 2 to the list. They're probably in the middle/high ground of difficult. You can die quite fast in those games, but can grind for armour upgrades to power through.
Probably not really accurate, but I'd go from easiest to hardest (not taking into account NG+ runs, which Nioh does really well with more content, bosses and loot). This is taking into account the various abilities the games gives you to raise your power to bypass "skill" checks. (The Surge games would be between Nioh2 and DS)
1) Code Vein
2) Nioh - Axe
3) Nioh 2 - Odachi
4) DS 1
5) DS 3
6) Sekiro
On another note, Elden Ring popped up on the PS5 store, couldn't pre order, but added to wishlist!
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