I don’t understand why you are asking about ideal credentials @ a CCC to get into UCD when he already has an acceptance there? If you feel UCD is a top engineering program, why not just go now? Holding out for Berkeley seems like a major gamble.
I second @aunt_bea’s concern, what if he goes the CCC route and then STILL doesn’t get into Berkeley?
Does he try for another UC? I would assume Davis would not want him at that point after deferring and then declining admission. Does declining admission last cycle to UCSD mean the same? I don’t know. What if he meets with academic or other challenges that affect his grades and prevent him from those credentials to get into a ‘top’ UC? Would he be happy at Riverside? Merced? Are there other non-UC options on the table?
In answer to your question about pitfalls, not every student who attends CCC in hopes of transferring into a UC has the stamina to make it through. Your son already has a touchpoint with the class he took at his local CC - what if this is more the norm than the exception? Friends with kids at our local CC’s complain that the majority of their classes are online so they aren’t meeting new people, that they can’t get into impacted required courses and thus transferring takes longer, that they are missing the ‘college’ experience living at home, etc.
Finally, per their own website, the CCC’s have 2M students enrolled annually, only 20k of whom successfully transfer to one of the 8 UC’s each year. While there are goals in place to increase that number, it means roughly 2,500 kids per year per UC transferring in from a CCC - and I’m guessing Berkeley does not take an equal share. Just food for thought.
FWIW we also loved UCD, though our student chose not to go there. It’s a great campus. If it were my kid Id say go with the sure thing and get it started! Love the college that loves you back!
Good luck to you all with the decision!