Wouldn't the game be a year into serious development at this point.
You think they just started development last year?
GTA V started
full production in 2010 after a two year pre-production period where Rockstar tried to figure out the scope of the game's open world. GTA Online and RD Online kinda changes the whole R* development timeframe as a good chunk of R* devs around the world have been working on keeping them updated with content for years now and the horror stories that came out in 2018 gave us the idea that a lot of time is dedicated in this department.
This is just me taking a shot in the dark looking at Rockstar and especially Rockstar North historically but preliminary work and planning on GTA VI probably took place following GTA V's launch (so in 2013, which is when voice recording and performance capture sessions for RDR2 started taking place) with Dan Houser helming it just like with RDR back in 2010. If not in 2013 then certainly after the next-gen version of GTA V was released so 2014.
Rockstar Studios (the combined power of all of Rockstar's subsidiaries) was used to facilitate the development of both RDR2 and GTA V after the launch of RDR. GTA V's next-gen port gave them a better idea of how to work with the Xbox One/PS4 and RDR2 continued development with work on GTA VI still preliminary. Developing RDR2, GTA Online and GTA VI at once is just too unreasonable to expect for any developer, even Rockstar. GTA V is still selling after all.
The small creative team working behind-the-scenes on GTA VI while RDR2 is in development probably came up with a rough outline for the game's story around 2014-15, extensive research is conducted in the real-world equivalent of the city the game is going to be set in. Skip a few years, maybe 2017, a casting call is sent out, Rockstar find their actors, some work begins on their end. Rockstar are familiar with the hardware by this point, maybe we get a basic greybox city just like with GTA V.
Skip a year to 2018 when Rockstar are wrapping up work on the base game for RDR2, the core Rockstar North team start to move onto GTA VI while studios like Rockstar Leeds and others work on maintaining GTA Online and RD Online.
The big thing that makes this hard is that when it came to moving from GTA IV to GTA V, there was no need to allocate resources to continuously update an online mode, but now Rockstar have two of these that are still receving excellent support to this day. It makes you wonder how this has affected GTA VI's development.
Obviously this is all just speculation going off of Rockstar's history and how development of GTA V was handled. At the end of the day, we don't know jack shit about how Rockstar operate so none of this really means anything. I'm not really expecting anything out of them for a good while though.
