Yes - when you do a renewal, the co-termination date will move out into the future. Bear in mind though that if you only renew the MXs you mention, your Dashboard will be out of compliance and you will go straight into the 30-day grace period. If you aren't using the MX250 and the wireless APs that you say you won't be renewing, you should remove them from the Networks to which they are assigned (you can leave them in the Inventory, unassigned)
Basically; check that the Current device count column matches what you want to keep working, after you apply the renewal and make sure the renewal license covers all of those items. Don't forget too - the MX licences must be Advanced Security ones.
If you do need the MX250 and the APs to carry on working, after the renewal you must include them in the order for the renewal licences. Basically - you can only Renew an entire co-termination Organization.
Any remaining time left on the devices you do renew will be rolled into the updated co-termination date.