Yes, I seem to agree with your statement that the newer bills are being trickled out at a super slow pace. This trend will likely continue even in the post-COVID era.
I also learned from someone that banks no longer order bundles or bricks of new bills due to escalating costs (after all, polymer isn't cheap), and the new bills are being released in smaller quantities, if mixed in with a majority of bills that were circulating for many, many years. If this is the case, that could explain my weakening interest in serial numbers over the last few years, though the only banknotes that are of interest to me nowadays are the $50 and $100 bills, especially when I can withdraw them from a Scotiabank ATM that dispenses multiple denominations and are located near where I live.