How would an on-chain signal work? Do you mean another proposal closer to the deadline? That is possible, although if it were created last-minute, some validators might not see it & confusion could ensue - I think a relative restart time is likely to be safer (unless there are problems that I haven’t thought of yet).
A second signaling proposal is safer if the first doesn’t include an exact time and mentions that a second will come, so that seems OK too (but this proposal does include an exact time and doesn’t mention a second signaling proposal).
I don’t have a strong opinion on window length, just a general preference for safety over liveness - network upgrades conducted like this are risky operations for validators & users alike, better if all involved have time to proceed carefully without worrying about downtime if they don’t export & restart fast enough.