I don't use CO2 in my tank but i do do a little home brewing (mead and beer). Applying my experience with that to your situation I might have a few tips/observations that could help(or not)

1. There are specialized brewing yeasts that are bred to withstand higher levels of alcohol. This could help with our longevity issues as a bakers yeast will die off well before yeasts bred specifically for brewing. Try Lalvin K1V-1116. You can get it from any online brewing supply store or check your local liquor stores.

2. The healthier the yeast the more it feeds and the more it feeds the more bubbles you get. Think about cutting back on sugar and adding some lemon juice (acid) and raisins (tannins) to the mixture for happier/healthier yeast. Grape juice has both acids and tannins so you could also add a dash of that to every batch.

Not sure how well this translates to a DIY CO2 system but any time I make up a batch I usually get this for at least a few days, then it slowly peters off over about a week.

Yes, brewers yeast is much better.