Current tires are a bit chunky at 38C and the guy at the bike shop brought up swapping in a larger crankset. Is this all necessary?

For the tires, it depends a lot on the quality of gravel/dirt, but generally you'll want to stuff the largest tires you can in there (going a little smaller than that is ideal for lots of mud and rocks that lodge in tread so it can clear the frame and not scrape the hell out of it but I digress). Running smaller tires, you'll generally have less cushion, things become more jarring as tires get smaller, and if you're on a bike for 10 hours, you'll want as much cushion as possible - it's a long time to be gripping the bars and sitting in the seat and you'll feel it more with 32-35mm compared to 38-40mm+. Also, skinnier tires, less traction, bike might feel a lot more vague on skinnier tires, might take a lot more input to control the bike, lot easier to have wheels slip, and if you're on a bike for 10 hours you probably don't want to be dealing with that kind of attentiveness - I run 35mm Gravelking SK+ mostly on hero gravel and I have wiped out so many times that I lost count, more often than not when I start to get a little bored and tired, front tire slips and next thing you know I'm airborne.

I'd put big tires somewhere between 'necessary' and 'use them until you're comfortable with them and then try more engaging tires that are smaller or have less knobs or file tread'.

As for the crankset, I dunno what chainring(s) you're running, but if you're going to be going on long outings in unfamiliar places, you're not going to want large chainrings. You might have small chainrings right not and might even be spinning out on smaller chainrings right now, but when you're hauling 30 pounds of gear and water, hills get a bit more challenging, and when you're hours and hours into a ride and dealing with fatigue, on top of the gear, and you're encountering hills that are quite a bit steeper than what you're accustomed to, you're gonna probably wish you had the wimpiest easiest gearing imaginable to get over that hill.

I would put a larger chainring as absolutely not necessary and probably a bad idea considering what you want to do with the bike. Unladen and on pavement, the gearing might be a little short and easy, but with gear and fatigue and elevation and gravel and dirt, it might be perfect or even too tall/hard.

tl;dr: 38mm gravel tires and short gearing is great, load that thing up and give it a go