First of all you seem to confuse flash space and memory.
The flash has 4000 MB which are split into ~1 GB for the root partition (even with picons this ought to be more than enough) and the remaining space can be used in the /data partition. Don't mind the few 100 MB that get lost in the GB to GiB conversion.
In Telnet you can check the state of the flash with df -h / and df -h /data
And as you can see on your screenshot youself. Right now you still have half of the flash empty. My 800se was always at 70-95% flash usage
Then there is the memory aka RAM. As you can see in your Screenshot the memory is also not fully available. This is because half of the memory is reserved for the GPU (similar as most Notebooks do it), which displays the OSD and other stuff (e.g. 3D Animations, Covercollection, etc.).
Regarding your "issues" with filling up memory: This is totally normal and no problem whatsoever! The default Linux memory strategy always fills up the whole available RAM and only removes stuff from the RAM if it really needs to.
Of course there might be ways to manually free up some memory but this shouldn't be necessary. Their either is enough space (and then Linux will use all of it) or there isn't (then you would need a SWAP). If you are not using any special software (e.g. Kodi, Full-Debian or other Desktop-Software) the remaining 1 GB will always be enough.
The command to check the RAM is free -m