Correct, they have cap holds so they could still build a roster. Right now the 2020-2021 Salary Cap looks to be $125M but we don't know for sure. But the Lakers also have that Dead Cap of $5 from Deng so even if the Salary Cap is at $125M it would be really tight with a max contract of Giannis and not likely at all.
Lebron is the only guy on the books right now for a player option. $41M
AD would be getting I think 5 years $200M - so $40M/year
As far as what the Lakers could offer Giannis...I'm not sure. Probably around $40M a year or little less. Not sure what the Lakers could offer him at this point.
At this point, it's not very realistic at all especially with the Salary Cap dropping. Staying with the Bucks, going to the Heat or Mavs makes more sense.
Next season is 2020-21. Unless I misread something, Giannis wouldn't be available for next season. But he will be a free agent after next season, so they'd be looking to sign him for 2021-22.
There's no way that the Lakers could clear the cap space to get him for 2020-21. But they might be able to for 2021-22.