I got my car wrapped soon after purchase, despite my initial thought that it was a crazy expense! The film used was a type that includes the ceramic-infused top layer and apparently does not need a subsequent topcoat. If desired, the liquid coating product can additionally be applied and they would have been happy to take my money for doing it, but they told me it wasn't really needed with this newer ceramic PPF. As far as I can tell it should last a long time as the ceramic layer is essentially bonded as part of the film, and not simply equivalent to a factory application of the liquid ceramic-coat product.
All the further comments should be taken as my impressions. I don't present myself as a great expert, just as a customer who went through a similar decision process recently.
In searching just now, it looks like Xpel's premium PPF is Ultimate Plus and I don't see that they're claiming it to include the ceramic layer. They have their Fusion Plus ceramic coating that they would obviously recommend as a companion product, but I think you cannot buy it as a retail product, it's for the shop application. I don't know enough to say that it would be any better for Xpel PPF then another quality brand of ceramic coat. You can buy an Xpel product called Ceramic Boost that is really intended to go over an already ceramic coated paint or PPF surface. It has good reviews including people who seem to have used it directly on PPF.
I also found a recommendation from 2018 that CeramicPro is the best coating on top of Xpel ultimate Plus PPF, but that was from a shop that obviously sold both, and I think a year before Xpel came out with their own Fusion Plus product.
Overall, my impression is that you could wait a while before doing any ceramic treatment, but the advantage of doing it right away is mainly that it would take a little less effort to clean the entire car before the application. Since you're in LA and you have a lot of sun, I would not wait more than a few months, in order to get the most benefit from the ceramic topcoat over the fresh PPF.
I think if I were in your place and you think the PPF guys are being straight with you, then you should just ask them if there's any downside to doing it yourself, and if you do are there any products which are more recommended, or products that are best to avoid in case you want their professional coating later. Also be ready for them to offer you a better deal on the coating when they sense that you're about to dtive away without it.