I have a rough cashing priority in my guide. Depending on what you have in mind, I may not be able to help. For example, without knowing the rates in the various revolving promotions, any advice I could offer would be pure speculation. As a rule of thumb, if I don't understand what I'm buying, I probably shouldn't buy it.
I've roughly listed the accessory buffs in order from highest to lowest preference in my guide. I have an Accessory Comparison Calculator for comparing combinations of rarity tiers and buffs, and an Accessory Enhancement Calculator for determining optimal leveling strategies (along with a few of my favorite recipes). It's not plug-and-play, but the tools to figure it out are there at least. I'll think about expanding the section and not leaving so much up to the imagination. I have a few other priorities in the next few weeks though.
Exploring Souls is a sort of rabbit hole, as Soul choice is very build-dependent. Add in the Relic weapons and it becomes a complicated and highly subjective subject very quickly. Quite frankly, I'm not ready to tackle it at this point.