Well, it really depends on what we mean by "custom design" and then ask if what's custom to PS5 is of no equivalent on the PC side of things. Because truthfully, in terms of API features, and CPU/GPU featuresets as a whole, even in terms of I/O API features, everything the PS5 does has an equivalent in some form on PC in terms of comparable SDK APIs, GPU featuresets, etc. It's not like there's anything revolutionary being done with the PS5 in a hardware sense that isn't supported by modern-era CPUs and GPUs WRT featuresets, and the I/O features of the system can be replicated through other combinations of hardware & software, like using the CPU or GPU for types of data decompression and the (slowly-coming) advent of API stacks like DirectStorage next year.
Truth is they don't need to develop for lowest-common denominator PCs because the LCD is always scaling upwards. A couple years ago the most common LCD for GPU cards was the 1060 IIRC according to Steam survey, which was basically a PS4 Pro, and that came out in 2016 (same year as PS4 Pro). In fact given the architecture differences it was more powerful than a PS4 Pro in terms of TF and certain other areas like pixel fillrate, and it's still the most popular GPU among dedicated GPUs for Steam gamers. Going by that example, it probably won't be that much longer until a decent portion of the PC players who'd buy these Sony games on storefronts like Steam have GPUs comparable to what's in a PS5, probably in systems with better CPUs to boot, and DirectStorage being active by then.
So the thing is, Sony's 1P are still going to target PS5 hardware for the baseline. However, most things are relatively easy to pare down through various settings, not just textures but also things like NPC counts, model density, physics systems, AI routines etc. Not all are scalable to the same rate, but there's flexibility. The same goes for how the games handle data I/O because, again, it comes down to the featuresets and there's no feature in PS5's I/O that doesn't have an equivalent in other environments on PC, even if the hardware to run the features and APIs to expose them might differ depending on user setup.
I suppose you can call that "custom design" in terms of the whole package but that "package" can be replicated through various means on PC. If the user doesn't have a fast enough SSD for example, they'd just probably have more system RAM that could serve a similar purpose. If their GPU doesn't have cache scrubbers, well that ultimately may not matter if their GPU has way more cache and runs just as fast, is faster, or has a notably wider design (because I suspect PS5 uses patrol cache scrubbing which would mean it's not the application itself doing it but something the OS is doing in the background when there is timing and resources available to engage it). Basically, stuff like that.
But, they'll always use their own PS5 spec as the target platform in terms of optimizing the experience, and if the majority of games are still having staggered releases anyway (1-year to 3-year, most likely), it gives them time to have a team optimize the game for PC, decide what setting can be lowered and how in order to open the game up for more system configs on PC while keeping general track of where the iGPU, dGPU, and CPU markets will be trending (that way they can assume what absolute minimum baselines there'll be, including which ones will likely be standard for core gamers on PC.