Isabella on a coastal start gives you the best shot at Buddhism or Hinduism.
You mean Isabella with a lake or a seafood tile in the BFC, right ?

That != coastal ... a landlocked start with a lake will do exactly as good as a coastal one with seafood in this regard
Early religions are quite iffy of paying out the investement you make in teching towards them vs the best of the other teching options ( normally something for workers or WB to do ), even if you start with mysticism. This is because having founded a religion will not have a big early effect ( shrines come far later, you only get the

bonus if you adopt it, and espionage bonus are far away of you mind ... ok, you got more culture in the cities with the religion and acess to temples and monasteries earlier, but the first is of little relevance most of the times and temples and monasteries are something you will both only have necessity and capacity to build some turns in the future ), unlike the option of building farms, mines, pastures, camps , cutting woods, working water tiles and make WBs ... so you are trading a early advantage for a later one. That in this game is normally a bad idea.
The diplo manipulation possibilities are probably a far more important factor in it, but that laso plays against founding religions early, simply because you don't have the necessary intel to decide if it is a good idea diplomatically to be the founder of a religion in t8 or 10 .You can found a mega love fest ( not always good for you ), a bitter cold war between the two early religions, a three kinddom religion board with judaism or confu as the third partner, your religion might be only spread to your cities and become a minority even being early ... in resume, there are a lot of scenarios, not all good for you , that you can be creating by your action, but they are shrouded both by the intel you still don't have ( it is unlikely you know everyone and everywhere when you start teching meditation or polytheism

) and by the various RNG interventions in this issue ( religious autospread , but also AI teching decisions, events, barb moves ... ). So , diplomatically speaking, founding a early religion becomes a huge gamble, just because you don't know what will come from that. and with the huge role religion has in this game, that alone is enough to kill your game if you chose unwisely .
And to add, there is very little diference between founding a early religion or capture the holy city from a neighbour of yours that founded a early religion .... and again, the advantage of actually having the holy city of a religion might be from 0 to huge depending of factors you have very little control
In resume: IMHO, unless you know the map before you start the game it is normally not worthwhile as a metastrategy to found a early religion. It may surely pay off in a lot of games, but in general it is just too risky for the average benefits of the move.