if your low flow toilet is 20 yrs old it's a first generation. They always were poor flushers even with perfect venting. The most affordable modern low flow is probably the cadet3 available at any big box hardware store. If it's like here they have a display with ratings for solids and water used. The difference is night and day and it's way cheaper then reworking your vent stack.
Thank you for that info. 👍 I was unaware of that and will keep that in mind about the cadet3 toilet.