It's really a case of diminuishing returns. The change for x:5 is
definitely affecting the logic of LilyPond, but arguably that can of
worms has been opened with x:13 already. x:5 is more important, but
it's also a lot more likely to be used as basic building block, like in
x:5.8. About x:5, I definitely feel ambiguous. In contrast, x:sus does
not have all that much logic hinging on it: it's previous behavior is
really "cute" in a programmer's sense of the word more than anything
else. Other modifiers also introduce "personalized" behavior (cf
c:dim7) and people are unlikely to have used it much, exactly because
x:sus is musically not anything suggesting a powerchord.
Maybe x:1.5 is tolerable enough. At any rate, the proposed x:5 is quite
analogous to the existing x:13. Your proposals for c:4 and c:2 would
require opening yet another scheme while the chords already have a
musically sound name x:sus4 (now also x:sus) and x:sus2. So the
threshold for x:2 and x:4 seems yet higher than that for x:5.