Many are very poorly maintained and would take months to get launch ready

It's the exact opposite of what you think. Nukes are the best maintained part of any military they are part of. Both Russia and the US have massively modernized their nuclear arsenals off late. The US has given all its nukes hard-kill capacity with superfuzes and the Russians have increased their potency by developing hypersonic missiles.

Ground based ICBMs are in fact the secondary means of firing nukes. The primary means are nuclear submarines. Britain and France both use subs near exclusively to deliver nuclear strikes.

ICBMs are very old and mature technology. Hell, even Pakistan and North Korea have mastered them to a degree.