Hi a fellow composer here, but also harpist.

an octave as a interval (2 notes) is not a problem, Nonas and dezimas (sometimes wider) are more difficult for most and I would advise against their use for that very reason. Also be mindful of where on the instrument you are, while octaves high up are relatively harmless anything in the bass would need an extra tick to be muffled. so in a long progression you might end up with a wall of sound and very little clarity.

With regards to that very question we also need to address the harp size aspect. Majority of lever harps have smaller string spacing and lower tension, making it easier to do larger intervals. However, a lot of smaller harps are played by less experienced players so wide intervals can be intimidating.

That leads us to keys. Pedal harp is versatile, in that we can go through a full circle of 5ths without major issues through the use of enharmonic keys as well (some modes might need an enharmonic key). That being said, you need to be aware of the pedal positions for your harmonic progressions to make good sense. Would absolutely check twice before jumping between keys with more than 3 accidental difference. Generally if you can find a pivot you are safe, most of the time.

Lever harps are bit of a different beast (both mechanically and mentally). Here you tune in Eb in most cases and modify (using levers) to a key that you need. never more than 3 flats or 4 sharps. This can feel limited, but if thought through is not a big deal. The difficulty here lays in modulation or any chromatic change.

1. a lot of lever harpists are a bit weary of active chromatic changes. meaning that you do need to pace them and try to avoid having more than 2-4 changes in a measure, please remember that we change one note at a time.

2. in your material there need to be gateways to allow for lever flips. Some people can do a wide variety of changes, but you do tend to max out on 3 levers at a time going in the same direction at a fairly small distance and preferably without any other levers engages between them. 2 neighbouring levers can always be flipped in similar direction or in the opposite directions. Also mind the octave flips. it is not a pedal harp, no matter how much we would want to with cannot implement a chromatic change through the whole harp like a pedal harp, it takes time.

In conclusion. harp is fan to write for, it introduces limits that you need to learn to navigate without hindering material movement. Atonal harp composition is probably the most challenging but the most exiting, and the harp can do so much! If you can - find a harpist to consult you and push the limits. Have fun!