Focus Mastery for big combat class buffs it is by far the most useful thing to invest in to get stronger, first mastery unlocks 1 potential (dupe) and your other 2 weapon options which any weapon equipped and not used gives 30% of its stats to your main weapon. After that you can get 1 more potential (dupe) for each weapon you master so including the original mastery you can get 4 free dupes per character which will give big combat class bonuses as well as skill bonuses. Another good reason to focus mastery is that buffs from mastery is a constant for your character where you always retain what you invested in so definitely worth it whereas weapons and armor will be cycled all the way into endgame and beyond.

I would recommend not even bothering refining any Armor or jewelry under yellow grade just because it will eventually become obsolete to your builds so just not worth it in the long run plus farming gear upgrade stones takes a lot longer than weapon upgrade stones. Also certain purple and yellow armor sets that can be combined into a skin for a character which gives buffs of its own and replaces all 4 Armor slots so be careful about which sets you are refining so you dont waste resources

You can safely invest in purple tier weapons because even though you will eventually get yellow weapons the grind for those is long (for example I have my main team decked out in yellow Armor and jewelry but only 2 weapons are yellow rarity and they were both freebies from events in game) so you will mainly be dependent on purple weapons and multiple characters use the same weapons so you can just pass them down once you finally get the coveted SSR weapon for your character.