If you want all 4x (S905, S905X/D) or 8x (S912) CPU cores to work then 1.5GHz is the maximum achievable stable CPU speed. If you reduce the number of active CPU cores you can (over)clock the remaining CPUs higher. Amlogic's 2.0GHz marketing can only be achieved with a single CPU core active, but this gives worse performance than all cores active at a stable 1.5GHz. Most vendor boot firmware lies and shows 2.0GHz at all times. Feel free to complain to your box vendor.
There are silicon bugs in the mmc controller on all Amlogic boards that impact on stability. There are workarounds implemented in the kernel, but the upstream kernel has rules on code quality so we aren't able to hack around problems as eggregiously as the vendor kernel sources do. SD card speed is currently set to 50MHz but can probably be raised to 100MHz reliably, maybe 150MHz. There are also additional SD card modes that can be experimented with. The vendor codebase uses 200MHz but that will not (100% guaranteed) be reliable.
The process for creating custom-remote configs documented in the wiki is still valid but the file format changed from .conf to .toml in a semi-recent v4l-utils update and the wiki needs to be updated to reflect that. Have a look at e.g. /usr/lib/udev/rc_keymaps/tanix_tx3mini.toml to see an example of the very lightly changed format.
Thank you so much Chewitt, awesome information. I'm not going to bother spending any time getting the oem IR remote to work. Great info for others though. My hardware is listed in the heading. It has an S912 octocore and LE reports them all as active. So far everything is working. Admittingly, I need to test much more.....not enough time to play around. However, I do want to mess with the SDCARD clock. I figured out how to use the dtc tool on my ubuntu PC. I'll tweak/optimize a dtb for my device and share it for others with the same device.