I will speculate that Lara will be elevated to the regular champion status by WBA. And if GGG is ready maybe next year then he can fight Lara but not right after his Canelo fight. He needs to take a break as well as you have said. If WBA is not keen on that, then Lara as champion will have his mandatory against the top fighters in the WBA ranking. And I think the IBF/IBO might be willing to give GGG more leeway as compare to WBA so he might retain that title after the fight.
Even if Golovkin wins against Canelo, I don't think that he should fight against Lara. From what kasabus posted, Golovkin will earn in the vicinity of $35 million from the trilogy fight against Canelo. A match against Lara would net him a tiny fraction of that amount. So I don't see any point in agreeing to a bout against Lara. Irrespective of whether he wins or loses against Canelo, Golovkin will be able to attract much better opponents. And a boxer who is very close to retirement doesn't care much about any of the belts.
If he will win, the call for another rematch might take place and that will bring them more money.
I see your point, the call for Lara is not that important and with his age he might rather to pick fight/s that will bring him
more and lesser with interest with another belt.
Though we don't know what is inside his mind right after the fight, but at this moment, for sure he's more on winning
the fight against Alvarez.