I'm IBS-A (alternating). Lately I've been more "D" and have been using Hyoscyamine, Bentyl, Donnatal, which has Hyoscyamine as an ingredient, and sometimes Lortab, which I have for another condition but seems to calm my nerves and lessen the "D". In times of extreme "D" I take immodium.
Just like you I sometimes have dizziness with Bentyl for about 30 minutes after taking it but it doesn't really bother me...I just never drive after taking it. Hyoscyamine and Donnatal can sometimes give me a headache, which actually does bother me.
BTW, did you know in some countries in the middle east they actually abuse Bentyl recreationally? I'm in no way advocating it but just when I looked up the side effects on Bentyl saw that and couldn't believe people were getting high off IBS meds.