Pixel 4a 5G and Pixel 5/5a hardware is indeed similar. I hope Pixel 4a 5G gets some extended support from GOS after November 2023. It uses Long Term Support version of Linux kernel 4.19. By 2023 (which is more than a year away), newer Pixel phone models will be affordable.
Those of us interested in privacy and security don't need the latest hardware. I have yet to see some feedback from GOS developers about Pixel's upgraded Titan M2 and Tensor chips in regards to security and/or privacy. I don't know whether GOS security tweaks make up for lack of newer Titan M2 and Tensor chips in older Pixel phones.