I don't believe the GT-I8730 is supported by PostmarketOS (nor Mobian).
Linux Phones
Community about running GNU/Linux on phones. Projects like Ubuntu Touch, Plasma Mobile, PostmarketOS, Mobian etc. Either on former Android phones or hardware like the PinePhone.
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Well then do you know if it could run any alternative OS, just for the fun Of tinkering?
Not sure if it will help, as I don't have a Samsung device, but I found this:
https://tweakdroid.com/lineage/samsung-galaxy-express-gt-i8730-expresslte/
I hope it helps, good luck.
Sorry, but this looks like one of many hundreds of almost identical pages where they just change the device and ROM names to create each new page. These instructions almost certainly won't work - typically aftermarket ROMs for a device only provide updates up to 2 or 3 versions newer than the Android version the device originally shipped with due to hardware requirements.
I believe OP will have better luck on xdaforums.com: https://xdaforums.com/f/samsung-galaxy-express-gt-i8730-roms-kernels-rec.5710/
You might be right. XDA is the tree that bears the most fruit in this subject, IMO.
Thanks, I’ll take a deeper look at this 👍