There's a lot to unpack here, so let's get to it.
First of all, Demon King of the Six Heavens is pretty useless. That's not to say that you should drop her right this moment, but you should realize that you're going to want to ditch her soon. She's UR rarity, which you might think is good, but that's actually bad, as it means you basically can't evolve her. You will occasionally get UR EVO or Universal EVO cards, but those are precious and need to be saved for use on much stronger generals, mainly Tiger of Kai and Master Strategist at the moment, though other very strong UR generals will probably release in the future.
When you're leveling up SR generals, it's important to focus on generals that have skills that will be useful to inherit later. That way, once you get your sixth general to level 55, you can immediately benefit from having more than five by inheriting a skill. When you do that, you want it to be a useful skill that you inherit, not some random garbage.
The SR general skills that will remain useful for the longest are the healing ones, mainly Naoe Kagetsuna, Honganji Kennyo, Ashikaga Yoshiaki, and Otomo Yoshishige. The +15% attack and defense skills remain useful for quite a while, too, though they eventually become obsolete due to the presence of Takeda Shingen of Kai. Many SR generals have only garbage skills and leveling them up is a huge waste of resources.
Longer term, the four generals that you want right now are:
UR Tiger of Kai
UR Master Strategist
SSR Sanada Yukitaka
SSR Takeda Shingen of Kai
You might say, oh, well those are hard to get. But so are the EVO cards to evolve SSR and especially UR generals, so save them for the generals that you really want rather than wasting them on less useful generals.
It's also important to realize that tanks are less important than you probably think they are. Once you scale up your damage, tanks and healing really end up only useful for beating reflect-heavy enemies, especially if you're trying to get an A for full evaluation. If your battles end in two rounds, that's not enough time to get much benefit from healing.
And battles do end fast, even at top end PVE. For example, the best non-convoy campaign farming is probably the sixth and seventh opponents of map 6-4, as they drop both curse fragments and gold tickets. Even there, on what is literally the very last map of the campaign, battles still only take me two rounds. If you're hoping to drag something out and win over the course of five rounds, you're going to be dead first, as your opponents have about 35000 attack.