Lots and lots of things happened that were not recorded. Generally speaking, if it was ordinary, it didn't get recorded. What got written down were the things that were extraordinary. So we can conclude, NOT that David and Solomon didn't consult the Sanhedrin, but rather that the rulings of the Sanhedrin during the days of David and Solomon were relatively non-controversial.
What is recorded is my original point. The book Acts is a record. The judges were for a time period of 450 years. After that the kings were their judges, which another record is seen in 1 kings 12.
Like other nations, there were groups assembled for consultation, an advisory to the king. But the king (as council president) ultimately decided a matter.
1 kings 12:6 And king Rehoboam consulted with the old men, that stood before Solomon his father while he yet lived, and said, How do ye advise that I may answer this people?
1 kings 12:8 But he forsook the counsel of the old men, which they had given him, and consulted with the young men that were grown up with him, and which stood before him:
Which acts refers to the council and all the senate... Senate which term has it root in aged (sene) (according to brittanica)
Ac 5:21 And when they heard that, they entered into the temple early in the morning, and taught. But the high priest came, and they that were with him, and called
the council together, and
all the senate of the children of Israel, and sent to the prison to have them brought.
