Ok, then I'll write my answer here, since I figured out.
You can do it on your Tablet without using the PC.
1. Download AOSPExtended For Lenovo A3500 (Android 7.1.2 Nougat) and TWRP 311 and put it on your SD Card (don't know if it works from internal locations).
2. Get root on the Device using Kingo Root.
3. Get the official TWRP app (link from APKPure here) and use it to flash the TWRP311_Lenovo_A3500.img you downloaded before.
4. Reboot into TWRP (pressing and holding volume up+down+power button), and flash the AEX-N a3500.zip from there.
This worked fine. Make sure to use at least this version of TWRP, didn't work with 2.x.
Didn't test the ROM too much, so far it seems to work for all basic things I tried.
I installed a new rom on my Lenovo a3500 and now whenever I try to access the file browser and gallery the no external storage pops up. :eek: I saw a fix on 4pda but I can't seem to download it :crying: 4pda.ru...