Technically, in a Linux X-Windows environment, you are supposed to be able to use xinput set-mode on the touchscreen device to change it to relative mode, but I couldn't get it to work on most of the touchscreen drivers I have tried, and the one time it did sort of work the result was unusable because there was no corresponding multi-touch and muti-tap support to properly emulate left and right mouse clicks and window resize and drag operations.
So, I feel your pain. Microsoft Windows has the TouchMousePointer pointer program which does work pretty nicely in full screen mode on Windows tablets, but sadly, there doesn't seem to be any equivalent touchscreen driver in the Linux world.
I know a program like TouchMousePointer would work really well in Linux, if available, because a few years back I picked up one of those cheap Z3735 Atom based Windows tablets and found that the only way I could run Linux was in a Windows based Virtual Box VM because Canonical did such a totally crappy job of supporting Baytrail devices (no WiFi, no touchscreen, no chipset driver, no uefi boot, and kernel crashes every five minutes). The Microsoft pop up keyboard was still pretty crappy, but I found a nice folio case with a nice mini-netbook sized fully touch-type-able bluetooth keyboard.
No way to get the Windows based TouchMousePointer driver working with the ARM based Raspberry Pi though, so your best bet is to throw in the towel for now and look for a nice compact USB or bluetooth keyboard/touchpad combo.
If it can't happen any other way, I wonder if the LoveSummerTrue guy who did the TouchMousePointer program on Windows would do a Linux version if someone did a Kickstarter or GoFundMe to underwrite the development?