Technically, is has to agree with all (or the phrase headed by all), not the list that follows. There are arguments for both singular and plural agreement. Some examples: all is well; all are present. In your case, it could be either, depending on whether you're treating all you could think about as a compound whole or as separate elements.
– LawrenceJun 16 '17 at 04:57
This has been addressed before, but in two parts. [a] 'All the X' may take a singular ('All the money has gone') or plural ('All the men were there') verb-form. // [b] notional override is often used (and, I'd say, often preferable): 'Bacon and eggs is my favourite meal. I'm so hungry that all I can think about is bacon and eggs.' With a coordinated list considered largely as a whole, I'd use a singular verb-form. But, with a list of items considered as separate elements ('Bacon and eggs have both risen in price this year'), a plural ...
– Edwin AshworthJun 16 '17 at 08:49
verb-form is required. In your case, I'd say you can justifiably choose how to emphasise. But if choosing are, I'd compartmentalise more clearly: 'are the sun, the oak tree, and the large bird.'
– Edwin AshworthJun 16 '17 at 08:49
You need a really good reason, and a clear exposition, to use a plural be in a cleft. The tense of the fulcrum of cleavage is normally irrelevant, since it's inserted by rule like it and there.
– John LawlerJun 01 '22 at 20:57