You will almost certainly lose autopay rewards for your next billing cycle if you remove your credit card or disable autopay. However, you'll keep your voicemail active and you'll be able to take your messages by calling yourself from another phone line.
The other option is to enable the "lost/stolen" feature on the last day of your plan (make sure to do it before autopay quicks in). As soon as you enable it, your plan stops working (no calls, no text, no data, no voicemail at all). The next day, no autopay payment will be made. Then when you come back you can reactivate your account and you'll get the reward since autopay was never disabled.
With both of these options, be sure to never let your account inactive/lost/suspended for more than 89 days, because on the 90th day you'll lose your number, any available funds in your account and you'll need to order a new SIM and create a new account.
In the end, you need to decide if voicemail for up to 89 days (~20 in your case) is worth $3/$6 (depending if you renew for 30 days or 90 days)...
