We do a lot of trailers in the shop. If you can use a 2x6 instead of a 2x4, do it. It distributes the load better. We use the slides if the customer requests it, but have very good success with marine carpet over marine PT (if available) or regular PT lumber, using stainless bolts and dipped-galvanized hardware.
Marine PT is not ACQ or copper treatment, it's the good old pressure treatment of old with arsenic salts. Much more durable, and not any more expensive, but hard to find sometimes. You have to ask for it. Regular PT typically lasts about 10 years, marine about 15, but with all of them you have to check bolt torque occasionally.
The composite bunk material is very good, and the prices were coming down pre-covid. Some require more supports than PT lumber, but they tend to last many, many years. Sometimes they break and we have to replace them.
Using raw, untreated lumber, even with some kind of sealant, is not a great idea unless you have a novelty boat that lives in a garage and only gets wet a few times a year. I've seen some beautiful bunks on racing and show boats.
The carpet you use makes a huge difference. Some carpet holds water and some doesn't. The 16 oz marine pyle from Sparta Carpets sheds water very well. Important to installing the carpet on your bunks is FULL coverage of outdoor carpet adhesive. This prevents tears and roll-ups, and probably helps prevents some moisture from penetrating the top of the bunks which is trapped against the hull.
1-1/2" x 3/8" S.S. lag bolts work fine if pre-drilled and torqued properly, but S.S. recessed T nuts are very good security. You want to layout and install the T-nuts before installing the carpet to the bunks.
Good advice given above about not letting your keel roller touch constantly. We end up welding quite a few cracked keels from that exact issue. Also, boats get holes in them from the brackets if the roller fails. Had one of those earlier today. Punched a hole on either side of the keel. Didn't think to take a picture, but the customer had someone weld a patch. It didn't leak, but boy was it ugly. We didn't do that work.