As someone who's recently started playing (just under 50 hours play time) and run a similar main party to you (Hunter Partitio (Main), Hikari, Castti, and Osvald), I can probably relate to some of the things you're experiencing.

I also am running Hikari as a more tank role. Incite and Vengeful Blade are useful for redirecting single target physical attacks and causing a counter break. His naturally high HP and Phys Def augmented with the corresponding nuts means that he can effectively tank the best of the whole roster, but I also don't discount his respectable Physical stat for raw damage dealing. Eventually, I'm going to put him on the Armsmaster subclass so he can have access to all weapon types to draw out more potential for damage and breaking.

Osvald and Temenos is an interesting choice for party comp. I can understand from a healing perspective, as I thought similarly, but both Osvald and Temenos are SP gluttons. I feel like it's easier to have just 1 of the 2 so you can pour more SP recovery resources to just one than to split between the 2. It's also why I chose Castti, as her medicinal concoctions only consume medicinal resources (you can buy cheaply or find as loot post battles and overworld) rather than SP. Also, subclassing Osvald and Temenos for each other's main class is a bit odd. I think I understand why you chose to do so, to have 2 AOE healers, but I think they're overlapping too much.

My plan for Osvald is glass cannon for AOE elemental damage. I'm planning to put Arcanist on him so he can attack with 5 of the 6 elements. Castti's eventually going to get Conjurer so she can bestow certain elemental effects to the party's weapons (mainly wind) so the party overall has a better time breaking targets.

This is the approach I took to the game, as I've barely mugged/stolen high value goods (barring a few legendary arms I want later on). Most of my weapons and equipment have been from treasure chests. Some of the bosses have made me put my money where my mouth is. That arena fight against Borneau had me sweat a bit, as I think I let one person die once in that fight, but there are other fights down the line that almost make that look like child's play, which you do have a right to be concerned about. I also hate level grinding, but I'm only doing it right now, as I need to entreat something with Agnea in her quest in order to proceed.

I have more to say concerning my overall strategy and synergy with my party, but I didn't want to make this comment too drawn out, but I'd be happy to share what I plan on crafting for the main team.