Purely from a logistical perspective, this would require a HUGE amount of work. You would have to make a complete emulation layer for both Android and iOS. While the former is freely available through Google’s own dev tools, I seem to recall people attempting the latter in the past being buried in legal crap from Apple. You’d also have to deal with the fact that the L5’s CPU and GPU are very weak compared to what you see in modern Apple/Android devices. That plus emulation overhead would make actually using this mockup device an exercise in frustration.
If you want to go the monitoring route, you’re best off sticking with Android - or rather, LineageOS. Pretty much all of the Google crap is gone from there except a few OS-level settings (eg. captive portal detection which you can disable). XPrivacy already intercepts and scrambles requests for tracking data, you should be able to make it produce a log with a few changes to the source (https://github.com/M66B/XPrivacyLua) if you want to document exactly what each piece of software does.
Apple - that one will be much harder both in terms of your coding effort and in convincing others.
Firstly because their OS is covered in both hardware/software and legal lockdowns - you’d have to get persistent root access on an iThing to be able to change what the OS says to software, their lawyers are quite trigger-happy and I don’t know what kind of documentation or examples exist for the OS’s core which would let you make these changes.
Secondly is because they have a history of standing up for the user (or at least appearing to) when it comes to things like spying and tracking - both with the highly publicised court case between them and the FBI, and also due to OS changes such as this one (https://arstechnica.com/tech-policy/2020/08/ios-14-privacy-settings-will-tank-ad-targeting-business-facebook-warns/).
If your angle is that of privacy and tracking, you’ll have an easier time getting someone to switch away from Google (because raping your privacy is the foundation of their business, though at least they have the decency not to use a rusty cactus like Facebook do) than Apple (who are actually helping in some aspects). The reason not to choose Apple (aside from price) - device lockdown - isn’t something that the average user would even consider.