I fully agree with you. I have not tried the Spencommand software, but now that you have mentioned it, I will try to find it and use it.
Apart from the comparative lack of customizability, there are 3 reasons which I find irksome about the 17.1 build.
First is the rotation bug. After a while, the tablet simply gets stuck in portrait mode and only a reboot gets the landscape mode working again.
Secondly, I need to swipe at least 3 or 4 times in order to pull down the quick settings and notification shade and it is extremely slow while it opens out.
Thirdly, I use my P905 a lot for making calls and in spite of trying everything, I'm unable to hear the voice of the person on the other line when on voice calls. I'm forced to use a Bluetooth speaker which is not always convenient.
The telephony aspect of this tablet is something that I am unable to provide any alternatives. My carrier appears to be incompatible with my tablet so I never looked into this at all.
In terms of the rotation bug, I use an app called Ultimate Rotation Control and manually rotate using its Notification widget. This bug exists with all versions of Lineage OS and this was the only workaround I've found which consistently works.
For the notification shade, this is one area that I think Lineage 16.1 offers a smoother experience. I found that certain apps like Youtube, TunedIn Radio, or any apps that utilizes a background image for its notification card will cause the notification shade to lag. Once an affecting app generates a notification card that causes the stuttering behavior, there's no way to recover other than to reboot. Even if the affecting app is closed, the notification shade continues to be slow and stuttery. The only workaround I've found is to disable all notifications for the affecting apps and then reboot.
In terms of quickly pulling down the notification shade with everything expanded in one go, there is an option in Lineage OS in the Settings app, under System > Status Bar -> Quick Pulldown. Enabling this option will allow a single swipe from the status bar to display a fully expanded notification shade with all the toggles and cards expanded.
I know the biggest problem for you is the telephony and call volume. Unfortunately nothing I have said is going to help with that. I hope perhaps Valera or a new developer may be able to help you. At least I hope my words would help anyone else with similar problems who happens to stumble upon this post.
Lastly, I'd like to share that to get the experience of a fullsize keyboard similar to Touchwiz, Hacker's Keyboard is probably as close as you can get with so little developers producing keyboards that are appropriate for tablets of this size.