Samsung Galaxy A6 - best open source system?

Could you advice which systems I can install on this device?
I do not see it on Lineage OS devices list, but mayby it can be install anyway or other system would handle it.

Other important question - if something went wrong would I be able to go back to default smartphone OS?
Is it hard to install OS on smartphone?

It’s best to figure out these questions on XDA or if you know Russian, (they sometimes have stuff that’s not available on XDA). Your first step should be to find out if you can unlock the bootloader.

Just because LOS does not officially support the A6, that doesn’t mean there isn’t an unofficial LOS port. From what I understand LOS has relatively stringent requirement for inclusion in the mainline supported device list; it’s not just a matter of driver/SoC support. Check the XDA forum for your device to find out what’s available.

In some cases you can go back to the stock OS (this can vary depending on the device). However, to install a custom ROM/OS, you most certainly will have to void your warranty. The ODM might still honour it; but there are no guarantees.

It’s not that hard to install a custom ROM, but it requires a certain level of familiarity with computing. The experience can vary significantly from device to device (it helps if your device is popular and has many ROMs/tools).

If there are no bootloader unlocks for the A6, the next best option is to get root access. Then you can control your system (firewall, content filters, custom app sandboxing) and get rid of corporate spyware. However, stock updates can be a massive pain if you get root. You could end up having to reflash an updated stock ROM (and then repeat all the root + config steps) every time there is a minor update.