As someone that was in a similar position last year (the first Inksgiving was last November and I had just started my comic the month before, in October) I would recommend using this opportunity to have support unlocked regardless. Even if you don't want to advertise or incentivize people to give you ink for this inksgiving, it's
A.) Nice to give your readers the option to if they want to. Especially for the event, every drop of ink to every creator helps!
B.) After the event convenient to have unlocked the rest of the year, as reaching 250 subs can be a daunting task.
Ultimately it's up to you, of course, but just some food for thought