I took a ~1200 mile bus trip about 20 years ago; it had multiple layovers and took about 36 hours total so it sounds very similar to the trip you’re taking. Frankly I would not do it again. This is from a trip up the east coast of the US so my experience might not apply in the country you’re going to.

I’d have to wait between 3 and 5 hours to board the next bus. Optimist me says great! I could go sightseeing, but with a large and heavy backpack this might not be a good idea…

If you’re using bus terminals in big cities, don’t expect to be able to get anywhere interesting and back again in that time. Large bus terminals are crowded (basically like a small airport) and sprawling and you’ll likely be fairly overwhelmed just trying to get to where you need to go to. They’re also generally not located in the middle of metropolitan areas; you’d most likely need to take a taxi to get to wherever you wanted to sight-see. It would not be a pleasant experience. If you’re considering this, check a map of the area around the terminal first and see if it’s even an option; don’t just wing it, and plan to need more time when getting back to the terminal than you think you should, especially if it’s a non-English speaking country.

Then there’s food, which at bus stations or in tourists areas is neither good nor cheap no matter where you are, personal hygiene, pickpocketing… I’d be traveling solo.

Expect to pay more than you otherwise would. Consider bringing food with you if you’re concerned about that.

are travelers allowed to eat in the bus? Am I allowed to bring my own food?

Yes, and yes, at least for the US-based busses I’ve ridden on. They’re pretty chill. Don’t expect to have a tray table or anything, though; don’t bring messy things. Something like a bagged sandwich, or snack items, though, should be fine.

The busses themselves were fairly comfortable, much moreso than a plane - the seats are well cushioned, you have room to move around. The ride itself was not bad. However, the layovers are killer. None of the legs of the trip were long enough to really sleep, either during them or between them; it’s hard to sleep on a bus (for me, anyway) - they’re not the smoothest rides, and sleeping at the terminals didn’t seem wise. I was a bit stressed about either missing my connection or someone nicking my stuff, though, so that added stress definitely made the trip worse than it could have been. I felt pretty awful by the time I reached my destination and just could not wait to be off that bus. Thirty six hours is a lot longer in practice than it sounds like when you’re planning things.