I'm not an expert, so I won't comment on everything. But I'll be happy to be the first reply.
One goal is definitely to minimize seams. With the ceiling height you list, horizontal placement is an option - you will just need to cut the edge that will be against the ceiling and the floor so you have a straight, non-recessed edge there. This approach lets you use longer panels to minimize seams - if that makes sense based on your room dimensions. (Or if you have the option to still buy them, since you mentioned you have everything ready to go.)
I finished a bonus room that had approx 8' ceiling in the center with sloping sections that were another 7' feet. I made the mistake of listening to the guys who were going to help me put up the drywall who said "you don't need a drywal lift" when it was time to do the ceiling. That was a big mistake. Lots of durabond to fix the gaps caused by slapping 5/8" drywall up against the joists and having to quickly get the screws in before they drop it. Rented the lift for the side slopes and it was the best decision I made. It was maybe $65 to rent for 2 days. Definitely recommend you rent a drywall lift when it is time to do the ceiling.
Assembles easy, lay the drywall on the outriggers, crank it up to the ceiling, jiggle into place, screw at your leisure. 5/8" is definitely heavier than what many of these guys are used to that are giving that advice. (They recanted it at the end of the day by the way, with the disclaimer they had always used 1/2")
Good luck. Are you going to mud it yourself? It is a good skill to learn, and if you're patient you can make it turn out very nice. If you can afford to outsource, it will probably be done a lot faster than if you do yourself.
I was planning to outsource, but our Golden Retriever tore both ACL's and his surgery used up my outsource funding. Was able to pay a contractor to help me get started and show me the ropes, and did it myself. Got it done, but it took much longer than if I had outsourced. But - I added a partition in my basement to wall off A/C and furnace sounds from the other half of my unfinished basement where my HT is, and was able to finish myself with the skills I learned. Turned out nice, and my budget was much happier. This of course, took longer than if I would have outsourced it as well, but nice to be able to point and say you did it all yourself.