Bear in mind they’re working on GTA VI and continuing to support the online modes for both GTA V and RDR2. You can’t expect them to do all that and remake GTA V in the Red Dead engine.
Okay, but "they" didn't need to do all of this themselves.
Rockstar North could have easily turned to Rockstar Leeds or Rockstar Dundee or an outside studio (or better yet, start up a new Rockstar studio specifically for remastering a few core titles*) and had them do most of the work (especially on GTA5; GTA Online still needs to be in step with the other versions, but 5 could have been redone a number of different ways by any number of studios and did not have to be on the same time schedule as GTAO.) We don't know that it is R* North actually doing the port work (hopefully GTA6 is their actual priority,) but IMO, the main Rockstar studio shouldn't be wasting any energy on this remaster when there are perfectly capable studios out there who could do it for them, just have an internal producer overlooking the work. I get that quality is important, but these ports were announced 15 months ago, still won't be out for another 6, and so far are not making a positive impression, so mission not accomplished.
(*
Rockstar Dundee is actually rumored to be making a GTA 3/VC/SA remake set, which sounds great; then put them on GTA LCS/VCS/CTW on that same engine, have somebody else do GTA4/L&D/BoGT, then have Leeds make a San Andreas Stories finally, and let's just keep rolling while waiting for GTA6...)