Remember the propaganda aspect of this war, using WMD's against well protected forces in the desert would not only be militarily ineffective, it would be a propaganda defeat for the regime. Saddam's plan has to be to maximize Allied and civilian casualties, get them bogged down in the cities, and hope the combination of casualties, with hostile domestic and world opinion causes a withdrawal.
No matter if Saddam's military is destroyed, or any other kind of damage, if he stays in power, or his sons if he is killed, it is a win for the Ba'ath Party, like in 1991 they survived.
Saddam considers he won in 1991, he lost Kuwait, with a large chuck of his military and WMD facilities, but he stayed in power.
To this hired thug from Tikrit, who got to power and stayed there with terror and killing his opponents, survival against a superpower is victory, he 'won' in his mind because he is strong, his enemies with their odd scruples about casualties, are weak.
If it comes down to fierce street fighting, and the Allies look to be prevailing, if the Republican and Special Republican Guard fall back, and/or start to surrender, that would be the WMD moment, nothing to lose at that point, WMD's more effective in a urban environment, plenty of dead civilians to blame on the Allies, civilians don't have masks or NBC suits.
Saddam thinks the US public cannot take 10,000 casualties, he may think they cannot accept far fewer, he's read about Vietnam, he watched Somalia, he thinks a bigger version on the latter, with the protests of the SE
Asia conflict that tore the US apart, is his trump card.
If WMD's can help bring that about, when things are desperate for him, if he can then still use them in any kind of concentration, he will.