I don’t have a coaching background, but I can offer you some feedback based on my experience with a similar issue.
As an FYI, the answer for you is going to depend heavily on your training background. I trained for my first Ultra after not having run a marathon before, and my best half marathon (on the flat) time before this was 2:03…
I decided to do the Ultra first, in October 2020 – the route was 50km, 950m elevation, and I ran it in 5:46. In May 2021, I did a 21km, 1000m vert FKT route in 2:21. I used the Luke Nelson Intro to 50km and the Mike Foote Big Mountain Plan respectively for the runs.
I think the key difference in the training approach is going to be specificity, and whether you can handle a particular type and amount training is going to depend on your training background. I was a very average runner when I first started, so training for the 50km route first allowed me to (i) build a strong aerobic base, (ii) develop general strength and (iii) sample with Z3/Z4 interval work without blowing a gasket.
I’m glad I did that first, because quite frankly I did not have the aerobic base (a lot of the Big Mountain Plan runs are run at Zone 1) and I did not have the strength (the ME workouts are ~ridiculously~ hard) to use the Big Mountain Plan effectively.
Accordingly, the Big Mountain Plan is going to be “best” for the type of route you’re doing, but whether it is the best plan for you as an individual is going to depend on your training background.
If you haven’t got that kind of background and this is your first attempt at a mountain race/ultra, then you could probably use the Intro to 50km Plan, but cut back the distance on the longer runs at the weekend and replace the distance with vertical (e.g. instead of calculating an 18 mile run, figure out how long that would normally take you and use that to figure out a steep route that takes a similar amount of time).
As always, continue to monitor your training load and cut back where necessary. The fact that it’s barely March and the race is in September does give you a lot of time, so don’t go nuts and try to add the volume in small increments.
Hope this helps and feel free to ask any follow-up questions.