It's going to be a learning experience for you HnP! People ride differently. Some ride 600 straight through with next to no sleep, or some none at all. Some take advantage of sleep stops or even book hotels. So nobody can really tell you what strategy will work for you.
However, as someone who will aim to ride pretty much straight through, I can offer my own thoughts. This 600 is extraordinarily well provisioned so, IMHO, it's an idea to take advantage.
In your bag drop, I'd have at least a towel and one change of bibs, socks, shirt. You could put two changes in, if you want to change on the outbound leg, but that'd be overkill for me - it's only 182km. Whether I'll shower and change on the return (433km), I don't know but at least I have the option.
I'll probably stick a couple of boxes of cereal bars in too, one for out and one for back. Not much else though. Remember, it's only a carrier bag permitted so you can't go mad. I'm used to carrying everything I need on a 600 anyway so I'm only using the bag drop to lighten my load, not to have more stuff!
I'll carry arm warmers (maybe leg warmers plus gloves), gilet, a shower/wind proof plus the usual first aid bits, munchie bars, gadgetry, etc. I've not looked at the weather forecast yet so my plan might change!
I'm really looking forward to this ride. Somehow, in my head, it's a bit different to the usual 600. I'm hoping to ride to enjoy it, take some LEL/PBP spirit with me.