It is true that a mist of aqueous sodium hydroxide can be hazardous to eyes or lungs (or other mucous membranes, even skin when present in large amounts for extended periods of time.) (It is not true that NaOH vapor poses any significant risk at or near room temperature, including at 65°C.) My first suggestion would be to stop the use of the ultrasonics, and try gentle stirring instead. If you must use an ultrasonic bath, then it should be sealed to prevent leakage of the mist into unprotected areas. There are many other chemicals which can provide a high pH.
I doubt your explanation that it is simply the pH which "dissolves" the PLA is correct. Na+ will react with PLA to form the salt in a de-esterfication reaction. It is possible that KOH may be more effective, either on a mass or a cost basis - you'd have to try it. The most obvious high pH chemicals are NaOH, KOH, and Ammonia (NH4OH/NH3(aq)), but you could also try lye, dry lime, CaO and Ca(OH)2. You could try mixed salts such as Sodium bicarbonate or sodium biphosphate to increase the metal ion concentration. There are also known catalysts for the desterification reaction, one well known one is iron (but many catalysts may discolor the plastic).
De-esterification also occurs in acidic conditions. You should consider the use of sulfuric acid or acetic acid (perhaps even hydrochloric acid) as well as the bases I've mentioned. In either case (acid or base), any residual contamination (due to poor rinsing or even absorption by the remaining part) can cause various structural and aesthetic problems over time. Be warned.
To answer your question in general (I do not have any specific experience with PLA), NaOH will be quite effective, it is unlikely that other possible bases or even KOH will improve your situation. CaO has obvious advantages in terms of both waste disposal and possibly for effects of aging, but it is also less soluble than K/Na hydroxides, so might be more difficult to rinse away, and it may not result in equally effective decomposition. (And note that your principle waste problem is the PLA waste.)