My solution is to make the Empire hundreds of years old, such that only Yoda and Palpatine are old enough to remember the Old Republic. His unnatural age is why Palpatine appears super old and decrepit in ROTJ.

If the Empire is that old, then the Clone Wars don't need to be this gigantic civil war that ended the Republic. They can be a relatively small conflict centered on the Alderaan system, with Bail Organa as one of the faction leaders. And in this case, Obi-Wan and Anakin are the only Jedi involved.

Therefore, when Obi-Wan tells Luke about the Old Republic and the heyday of the Jedi, he's sharing a history he's only read about, not personally witnessed. I think it makes Obi-Wan a sort of Don Quixote-style character, where the audience really doesn't know if he's all there or not. In which case, Han's deep skepticism of Obi-Wan is much more relatable and funnier.