If anyone's interested here's [0] a Mediafire archive of Symbian developer sources of all kinds, from compilers and SDKs to manuals.
[0] https://www.mediafire.com/folder/79jhy594xb3uk/Symbian_Devel...
[0] https://www.mediafire.com/folder/79jhy594xb3uk/Symbian_Devel...
I ended up doing mobile development anyway, but not until Qt was a viable option for Symbian.
Here is a list of active Symbian software developers* (+ links to its projects).[2]
P.S. I'm luckily archived/mirrored LCG's X-plore[3] & ProfiMail[4] apps source repos in time when it was released into Public Domain by LonelyCatGames* team (now official repos is disabled).
[0] https://mrrosset.github.io/Symbian-Archive/index.html
[1] https://github.com/mrRosset/Symbian-Archive
[2] https://github.com/mrRosset/Symbian-Archive/issues/10