Chemical weapons give a casus belli for the Western governments to
increase their presence on the ground
That's true that the use of chemical weapons galvanizes the west against Assad, but, I think the best way to put it is like this. "They're going to hate me anyway, I may as well go all out".
There's a body of people in the west who don't want war no matter what. They don't want to be in Syria. They aren't going to change their minds based on some rarely shown news clips.
There's another body of people who are anti Assad anyway. Hating him more won't change much. The actual shift that happens when chemical weapons are used isn't all that much. Granted, it's affected Trump and lead him to drop some bombs on an airport (after giving the Russians a head up) and it lead Trump to make a threatening tweet, but for the voting american public the use of chemical weapons doesn't move the dial that much. It moves it some, but it's hardly a "Casus Belli". It maybe could become that. So far it isn't.
Regular weapons are just as effective at assaulting the opposition
That's false. Gas is heavier than air. It sinks into bunkers. People who are underground run outside to escape the gas. Chemical weapons gas is also very painful and it hits a lot of women and children. It has an effectiveness that regular weapons simply do not have. It's also very good at breaking the spirit of the resistance.
Assad has been gaining ground ever since Russia began their campaign
there and it is only a matter of time before he wins the war
I don't know. He certainly is wining. I don't know if it's a clear cut case of "only a matter of time before he wins". Resistances can last for decades. One could also make the argument that Assad is impatient or perhaps make the argument that he's cruel, or that he's a tactician who understands the devotion of his enemy. To get into specifics would require a lot more knowledge of how determined and entrenched his opposition is.
So what rationale could the Syrian government possibly have for
resorting to chemical weapons?
I've covered this above, but just to summarize;
1) to break their spirit,
2) to punish,
3) to hit people in bunkers.
Whether you believe Assad is behind the numerous chemical attacks or not, there are clear benefits to him using them (there are risks too, but to say there's no benefit is flat out false).