What I hear you saying is
- You want to take your riding up a notch, start doing longer rides,
- You are drop-bar-curious, and have not been put off by them in your test rides.
- You don't want to throw money at this unnecessarily, but are willing to spend more if you get a better bike that you will not outgrow.
So my $0.02 is.... get a drop bar bike, and do not be afraid to put $1500 into it (the cost of the Checkpoint). In the world of road and gravel bikes (especially the latter) this is a very reasonable price point for a bike that has what you want and is a long-time keeper. There is a difference between a $1000 and $1500 bike that you will notice and appreciate over time. It sounds like you are riding enough (even on a crappy bike that does not fit you) that there is little risk of this bike sitting in the garage collecting dust. You will get your money's worth.
As far as getting a gravel bike..... YES. A bike like the Checkpoint is IMO a great all-round road bike. Good for pavement, rails to trails, dirt/gravel roads, you name it. And it can take a rear rack. I know you were talking about n+1, but with a bike like this you will probably never ride the old one.
Regarding disc: I don't think they are a necessity in and of themselves. However, if you are looking at bikes that clear 35mm tires (which IMO is a good idea), disc is starting to become the default, though there are a few canti models out there if it is important to you.
Good luck!