Another update:
I have a basic webserver running that reads out and displays some (currently 4) 1-wire temperature sensors, is able to switch a relay, and handles some configuration settings.
My PI zero now has Raspbian Light installed - very compact; I don't really need the GUI for my application.
I switched from PascalIO to Raspbians pigpio package and now handle the GPIO pins via shell calls:
uses ..., Unix, ...;
[..]
function SetPin(pinNo, State:integer):longint;
var
st : string;
begin
st:=format('pigs w %d %d', [pinNo, State]);
result:=fpSystem(st);
end;
[..]
That may be much slower, but - as I want to react to a weather forecast - split second timing is not an issue. The function above can do about 130 changes per second on a Pi4; more than adequate.
The calls to "pigs" do not require root privileges and debugging from within the IDE works fine.