Hardest and most time consuming for me was picking out all my light fixtures and plumbing fixtures... but mostly that was because I was trying desperately to find things I liked at the lowest possible prices.

I knew what I LIKED and had already picked out almost everything before we ever signed our contract with builder. But by the time we actually started buying lighting and faucets, our budget had taken a severe beating due to a host of issues with our builder. Some very very judicious on-line shopping allowed me to find alternatives at substantial savings that I liked just as well or better than my original choices. But I probably spent a couple of hundred hours looking. By that point, I had taken over GCing our build so I KNEW when I would need to have various items on-site and never got hit with any "make a decision by tomorrow" kinds of things.

Your floor plan and your exterior design are as linked together as your floor plan and your furniture arrangements. They are not seperate issues, they are one consolidated consideration. In many cases, they all require study and adjustment so that exterior, functional locations and adjacencies and furniture/life-style considerations are all working together satisfactorily for you. I hope this is the case with your "settled" floor plan.

As for challenging post-start decisions, these often include areas where the builder may have provided "allowances" for owner selection. These usually include paint and finish materials (cabinets, counter tops, etc.), appliances, plumbing and lighting fixtures, and the like.

If I were you, I'd begin to develop my preferences for these selections, balancing personal choice with expense. Starting early, as you are, will save worry and hassle and let you enjoy the process. This site and Houzz are good sources of information and experience to draw from. Start a file for each area of consideration and stuff it with your idea examples, sources and prices! Good luck!