Larian has been pretty clear that they are not going to make a DLC. I still hold a little hope for perhaps an adventure pack but even that seems unlikely.
What do you think, realistically will we see Larian put any more real effort into "upgrading" the game or are we down to a few more bug fixes and that is it?
I really don’t know. Definitive Editions about a year after games release has been Larian’s usual MO, but in the past that came with console ports - with those being done, it is difficult to guess if Larian is still working toward something of that sort. Theoretically, they might not have an incentive to do so, as the game is already available for sale on all platforms.
They said they had no plans for DLCs, but that was before the game sold like it did. The success of the title might have encouraged them to do something more, but even if they started to work on something after BG3 release, it would still be a long way out.
It sounded like Sven has moved on to the next project, but it is not impossible for a team to start developing a new project(s), while another team keeps working on and improving BG3. I can’t begin to guess, though, what further plans for BG3 might Larian have.