How much enjoyment would he get from books containing only the sounds that he has learned in phonics?
Many parents report that their children do enjoy 'decodable' books, and also the gratification that comes from being able to read them by themselves. No 'reading scheme' book can compare with a real book, but they have been found to be useful in teaching beginning reading.
And at what point would he start to recognize high frequency "tricky" words that are not decodable, even with phonics knowledge?
Almost all high frequency words are decodable, given sufficient phonics knowledge. The term "tricky" refers only to the portion of the word for which the child does not yet know the letter/sound correspondence. It doesn't mean 'not decodable'.
My daughter is also in reception and through bringing these books home, she now recognizes words such as "the" "wasn't" "said".
Many children can learn to do this, without it harming their reading. But a significant minority will become confused if told to use this as a word-identification strategy, and will also not acquire sufficient practice in decoding, if they are instructed to learn words as whole, or to guess from context cues and pictures. This is why psychologists who are experts in the evidence for reading say that children should be taught via decoding only early on, so that those with a tendency toward dyslexia will also learn to read well.
and uses context clues to help her when she comes across a completely unfamiliar word that she is unable to decode. All of these are essential early reading skills
The idea that using "context clues" is an essential early reading skill was based on unproven speculation by theorists in the 1970s, and there has never been any reliable evidence to show that it is true. It was nonetheless widely promulgated by the now abandoned "Searchlights" strategy that was part of the 1997 Literacy Strategy, and unfortunately many schools continue to believe what they were told then. As I said above, for some children it causes confusion, and distracts from the important task of learning to decode, which is why it is best avoided. While context is important for identifying homophones, that is a limited use, and children should not be encouraged to use it for other purposes.