Accelerometers are level sensors too. Built by semiconductor technology, they are rugged, small, cheap, sensitive, low-noise, and the electronics is often integrated on the chip or in the package. Just check if their zero is stable enough.
Bubble sensor: I saw one with electrodes three decades ago. Its development was already old. That company developed it for its own needs and didn't sell components.
The difficulties facing such a sensor are tightness and corrosion, which appear after years hence are lengthy to observe. My suggestion would be: don't start that development if your company affords only few months work.
As liquid-tight contacts through an insulator, I know metal-ceramic sealing, and nothing else: epoxies are corrosive, and O-rings don't give the electrodes a stable position. But you can find such feedthroughs already made, with a thread to fit at your container: car spark plugs are an example, feedthrough capacitors an other one. One difficulty is that this leaves little choice for the metal, often molybdenum, and such metals don't resist corrosion well. Cover with gold maybe?
I'd largely prefer a capacitive scheme, with the electrodes evaporated on the glass container. No corrosion, no leaks, stable positions. You can reuse an existing level and experiment with aluminium foil, with metallic adhesive tape (exists at 3M), or with conductive glue used as a paint.
In any case, the reading must be AC. DC voltages drift too much with time and corrosion. AC too needs enough electrodes so some inject the current and others measure voltages.
A machine washing liquid could be a start material. They have anti-foaming and anti-corrosion properties. Water-based hydraulic fluids, especially with polyglycol, are an other start. They have wetting, defoaming, anticorrosion agents. Both present a signifiant conductivity. If adding ions for conductivity, not NOT take a chloride not bromide, as these corrode metals. A phosphate would be better, or just a plain base. However, most anticorrosion agents work by creating a stable layer at the metal's surface, and this goes against a stable metal-liquid voltage. One more reason for AC operation.
Reading the bubble's position optically is horribly more complicated, but it's cheap since bubble levels and PC cameras exist already. Add software.