Is It a Subscription?
No. It is a single payment, and it keeps working without an internet connection.
No. One payment, and it is yours. There is nothing to renew and nothing that stops working if you forget about it for two years.
This is a job you do once. Charging monthly for it would mean charging you to keep access to photos you already own, which is the thing you are trying to get away from.
It runs on your own computer and is free for your first 500 photos. Unlimited is $9.99 once, on all three platforms.