There should zero emphasis on cpu upgrades until the pre-orders are fulfilled.
If I'm understanding things correctly the RK3568 is effectively the more hobbiest friendly version of the RK3566 used in the Anbernic RG503 & RG353P. Linux development thus can steal off the Anbernic related efforts, and GPU of the RK3566 is Mali based for what that's theoretically worth.
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I don't own examples of and have not detailed tested an OMAP5 & RK3566 side by side, but I would tend to expect CPU performance to be more of a side grade than an upgrade. The Cortex A15 is a three-issue Out of Order performance core from 2012. The Cortex A55 is a dual issue In Order efficiency core from 2017, intended to complement the Cortex A75.
www.anandtech.com
The RK3566 should be more power efficient, offer newer APIs, and offer a few other niceties associated with being newer but... while the branch prediction engine is far newer the Cortex A55 is still a two-issue In Order processor with an expected clock advantage of only 20%. In CPU terms I would tend to expect which one comes ahead to depend on the nature of the particular program, with something analogous to an AMD K5 versus an OG Pentium. That said the RK3568 should be fine for running Duckstation at 2x resolution and otherwise maturing up what the Pandora could with a reasonably efficient passively cooled solution.
Given how overdue* this project has been, which in part is due to the larger Linux Community not having developed particular interest in the OMAP5, I'm fine with them effectively leaning on JELOS and the FOSS Mali driver efforts personally. My only potential concern with the RK3568 would be the sound system side of things, given TI parts have been noted to for their good sound chips.
*
https://en.wikipedia.org/wiki/DragonBox_Pyra
Friendly reminder the Pyra was presented during the 2014 FOSDEM, and the original expectation was that it would be shipping around the same time as the original GPD Win in 2016. It's now 2022.