I'm looking at doing a decently long bikepacking trip mostly on road. I currently have a carbon framed orbea avant endurance bike. I'd like to do the ride without necessarily buying a new bikepacking bike. I was thinking that a seat post bag, frame bag, top tube bag, and maybe a backpack to ride with and putting 28 or 30 mm tires on. Does this seem reasonable as a setup? I've done century rides on this bike before and it's comfortable enough. What are the drawbacks of a carbon endurance bike vs. a more bikepacking focused steel or aluminum bike? I know if the carbon cracks I'll be SOL.
Yes it's reasonable, assuming your bike is in good condition when you leave home. Frame failures end most trips regardless of frame material (the bravado of "A village mechanic welded my frame" rarely comes to pass). Lots of people are bikepacking on carbon bikes--carbon frames have even accelerated the bikepacking trend because they typically lack rack mounting eyelets.
That is a reasonable setup and is a great way to start! Regarding the bags, protect the carbon frame with tape of something like electrical tape or helicopter tape anywhere the bags and associated straps touch the frame. See Bikepacking.com's great article for more information.