Yeast will not be affected by pressure, I have experimented a fair bit with DIY set ups as I keep both planted tanks and have made wine so this is my penny's worth for what it's worth.
The problem with DIY and plants is how unstable the fermentation process is which leads to unstable CO2 which in turn can turn to algae problems through fluctuating co2 levels.
However there are certain things that can be done to smooth out these fluctuations depending on your choice of diffusion.
Firstly the recipe itself, I use 200 gram of sugar mixed in 1 ltr of water with 1/2 tspoon of bicarb depending if you have soft or hard water. Leave the bicarb out in hard water as in soft it's only there to buffer up the mix which can get so acidic it can kill off the yeast (co2 is acidic)
The reason for the 200g in a litre is because more sugar will not be used as if this fully fermented out the alcohol content will kill off the yeast wasting sugar plus over loading the yeast with too much can cause the ferment to stick.
The fermentation process has three phases, first there is aerobic( in the presence of oxygen) then anaerobic (without the presence of oxygen) then the tail end. The only part we need to use is the anaerobic as the the other 2 create very slow co2 production to the point of being pointless.
Fully fermented out syrup with this SG will produce a wine dry (no sugar) with an alcohol content of roughly 10% anything higher takes a long time a specialist yeast which is no interest to us.
How to smooth it out IMO, You can use two bottles and start the aerobic part which last roughly 24 hours on the second bottle as the first bottle is starting to come to an end so that when you connect it up to your system its already going into anaerobic. Another method I use is rather than start a totally fresh batch syphon off the majority of it leaving the yeast sediment in the bottom of the bottle with some of the wine and just add your new syrup to that. This yeast is already working anearobic so the ferment picks up quicker and if you can do this late evening when the co2 is not needed so the next morning it should be getting back to full strength.
The reason I mentioned how your diffusing the co2 is because if you are using the glass ceramic diffuser they need a fair bit of pressure to get going which will delay the co2 going into the tank further, other methods like spiral ladders intakes of filter that don't need pressure built up are quicker at getting the co2 from the bottle to the tank.
Another problem with the glass diffusers are one tiny leak and the pressure will drop enough to stop it working which is not a problem with the other methods I mention.
As for sweeteners like saccharin a lot of wine makers add these to over dry wines post fermenting to sweeten them up because they won't cause secondary fermentation busting your bottles like you have mentioned so I'm guessing they won't work.