Congrats on the .com sale, that's a solid exit even if they grew huge later.
Regarding the .io: Since the brand is now established and worth $100m on the .com, the .io has transitioned from a 'brandable asset' to a strictly 'defensive asset'.
It’s hard to sell to a third party now due to potential TM issues with the main company. Your best bet is selling it to them for brand protection. Realistically, I’d value it in the low to mid $x,xxx range as a defensive acquisition for them. They might grab it just to redirect traffic.