Looks great, and there's nothing much more satisfying than building your own shop. All your outdoor equipment will thank you. I built mine last year, 24x36. Pretty amazing how fast you'll run out of room in that. I promise, the missus will find LOTS of things that need to be taken out of the house and put in your shop for storage. Get it finished inside so you can insulate it before winter. What I did for interior siding was break down pallets and trim the edges of the slats to use for the walls. It was a lot of painstaking work to fit it all, but it gave the inside a really cool look. I put a few sheets of plywood up on each end, but most of that was recycled crate walls as well. I spent my budget on the structure, siding, and roof because materials prices skyrocketed during the build. I got the pallets for free and they were mostly oak/poplar pallets, so I got some pretty nice wood out of all that.
Despite putting a 12" thick footing around the edges with TWO rebars for reinforcing, AND 6x6 re-wire under the whole slab, I have some fine cracks now. I'm thinking the concrete was crap, and it finally shrunk enough to crack. It took nearly 4 days for the slab to set good enough to start building on. The contractor I hired to do the finish on the slab got here at 7AM, and didn't leave until nearly 2 AM the next morning waiting for it to just harden enough to be able to float and finish it.