The fili strikes look good! The cape is still nice and green. I don't know if those dark spots are cells joining together to start strikes, but there's still good chance you'll get strikes soon on that leaf, too! I only tried water propagating leaf cuttings once. I absolutely didn't have a clue what I was doing and my attempt failed. I'm thinking of trying it again this year.
I don't know when to tell you to transplant them to a grow mx, but I would think you'd want them to have roots forming. What growing medium are you planning to transplant them to? Will you section the leaf where there's one or two strikes per section?
The growth of my leaf-cuttings from last year have *FAR* surpassed that of my seed-grown seedlings. A big difference, though, and may be contributing to the leaf-cuttings' great growth (as compared to the seedlings) is that the leaf-cuttings were started and have been growing in a mix of dried LFSM (lower layer) and live sphagnum moss (top layer) while the seedlings were started on a peat:perlite mix.
A wildcard in the comparison of the growth of my seedlings and cuttings are fungus gnats.
The leaf-cuttings (filiformis) were started outdoors on the sphagnum moss mix and had basically established themselves before moving them indoors. The seedlings were basically started and have grown indoors only...where fungus gnats have been present. So, I'm wondering if the seedlings have been hit hard by the gnats.
BUT, to throw yet one more wrinkle in the difference of the growing of these sundews... I've got a few cups of yet a couple more different filiformis seedlings growing in all LFSM (dried only, no live) that were germinated and having been growing in the same indoor conditions as the others...and they're out growing the ones sown onto the peat:perlite mix. So that brings me back around to wondering if the peat-based mix is ultimately the reason for the slower growth. I think the next time I sow sundew seeds that I will do a three-way side-by-side-by side experiment....perlite:peat versus dried LFSM versus live sphagnum moss over dried LFSM. All pots in same water tray.