Apologies for the slightly ambiguous title, I'm not sure how to word it.

I am looking to book Bangkok to Manado (BKK-MDC) which will go via Jakarta, in late April 2018.
These flights are still unavailable to book, although I already have managed to book a May 2018 domestic (Manado-Denpasar).

Looking on Google flights, and double checking on the Garuda website;

• BKK - MDC is an astronomical price on Google (£1200+ one-way), and when trying to book on Garuda it comes up as unavailable
• BKK - CGK is not yet bookable on Garuda (Google = £915 OW)
• CGK - MDC is bookable and of a reasonable price on both Garuda and Google

So, after a lot of preamble, my question is;
Does anybody know how far out Garuda open up booking for regional flights?

Whilst I could take multiple airlines, I want to do it all on one ticket as I will be travelling with baggage, and would like it checked through. Other options have included an overnight in Singapore, but I'd prefer to do it in one day, and not have to pay for a SIN hotel.

I shall also be asking Garuda on their Twitter feed.

Thanks in advance

Sean