is an SD card mounted with setuid mode? Related question here which gives all the gory details (that I won't repeat in this question), motivations and context.

Android is a Linux kernel based system. I know quite well about mount(2) and mount(8) and the filesystems on Linux.

Can the SD card in a tablet be mounted (e.g. as an Ext4 file system) without the MS_NOSUID flag? If yes, how?

I am willing and capable of writing, cross-compiling, and building a statically linked ELF executable (for my ARM-based tablet), which is root-setuid and mixes the features of sudo and sash on my Debian system. I am capable of formatting some SD card (on my Debian) as an Ext4 filesystem. And if I know how to install and run it on my tablet, I will write that program as a GPLv3+ software (in C).

A related question is what exact cross GCC compiler should I use on Debian/x86-64 to target such an ARM tablet.
External storage (/sdcard) and physically external storage both are mounted by vold with nosuid irrespective of the filesystem, can't be mounted other way without root.
Both storages are accessible to apps as emulated filesystem (with fixed file permissions) or through SAF; both don't let files be executed.
You can move ELF executable to somewhere on /data e.g. to /data/local/tmp/ using adb shell or to /data/data/com.termux/files/home/ using Termux app. /data is formatted as ext4 or f2fs, lets file permissions be set, so you can execute files there. But that too is mounted with nosuid by default.
Android apps are run with all Linux capabilities dropped (empty bounding set and NO_NEW_PRIVS set), so they can't make use of setuid or file capabilities to elevate their privileges.


You may want to have a look at How Magisk works? or How to manually root a phone? to see how root works on Android devices, not possible with locked bootloader.