Hm, so even airstrikes on, say, chemical weapons dumps would be out of the question then?
Any interference by Israel of any sort has as its most likely outcome the ending of the civil war and the solidification of Assad's control while causing his policy towards Israel (which is a sort of
détente with a smidgen of rabid-yet-cordial hatred that provides both administrations with bogeymen that are valuable for their internal PR narratives) to harden.
Except actually killing Assad, which has as its most likely outcome the ending of the civil war with a hardline Islamist group in charge which will not display Assad's restraint when it comes to Israel - note, for instance, that when Israel attacked the Syrian nuclear programme, Assad did not counterstrike Dimona, say. Not so say that a Syrian missile/air strike would be successful - I mean, there is a reason the IDF spends all that American aid on those cool toys - but it would be really embarrassing if one unlucky Scud in a salvo overwhelmed the area defence systems and ended up cracking a reactor vessel to make a big chunk of the Negev glow in the dark.
The plain fact is that whatever Israel thinks of Assad, he's hamstrung in his own actions against Israel while he's fighting a civil war, and it's really unlikely that deposing him will result in a regime which is as positive* towards Israeli interests (including shenanigans in Lebanese politics) as Assad's one. Which is quite a depressing thought, if you live in Tel Aviv.
* Well, maybe
not even more overtly negative would be better phrasing here.