Hi,
thanks @deckingman for your reply, and sorry for the delay, I only had time to get back to this today.
I wanted to try the firmware retraction and pressure advance and started with the pressure advance calibration script posted here in the forum. (The script prints a single wall along x with 5 sections of 100 mm/s connected by 4 sections of 5 mm/s moves and changes the pressure advance along z.)
With the bowden tube, the initial 0 to 0.2 range was not enough, so I did a second run with 0.2 to 0.4 and with that one I got an intact wall for >0.3 s.
However, I still have quite some oozing problem -- the 5mm/s parts are way thicker than the 100 mm/s (almost 3x) and i get the 'oozing/layer seam' at the one end of the wall when it does the layer change. (I did not use any retraction setting for this/it is not part of the script -- anyway that would not help with the wall thickness.)
I though this might be temperature related then ("the filament is just too fluid and just run out of the nozzle"?), but I see very similar results both with 220 and 200° for PLA and I don't dare to go much lower (I had some clogging issues at 190°, maybe that was just coincidence).
This would also fit to my observation above, namely that the retract (+negative extra length for unretract) setting did not affect the ooze seam. There just seems to be molten filament in the nozzle which runs out, no matter what -- could that be?
One more observation: When I just let it extrude a few mm of filament (with the nozzle away from the bed), there will be a delay before filament is extruded and then there's filament running for up to over a minute (e.g. with 10 mm at 1 mm/s, it was coming out for at least 90 s).
If anybody has a suggestion how to tackle this systematically, I would really appreciate this -- I feel like there are too many parameters at the moment.
Also, if anybody has some patience to "sanity-check" my config.g? 
M669 K1 ; Select CoreXY mode for Firmware >2.03
M555 P2 ; Set firmware compatibility to look like Marlin
; Motors Part 1 -- Define motors
M584 X0 Y1 Z2:4 E3 ; must come before M350, M906, M92, M201, M203, and M566
; Axis Limits
M208 X0:270 Y0:220 Z0:250 ; "min":"max"
; Endstops
M574 X1 Y2 Z1 S1 ; Define Endstop positions (0: none, 1: low end, 2: high end), S1 = Set active low
M558 P0 ; z-Probe (P0 = no z probe present)
; Network
M550 P"V-Core 2.0" ; Set machine name
M552 S2 ; Set Wifi mode to 'access point'
M586 P0 S1 ; Enable HTTP
M586 P1 S0 ; Disable FTP
M586 P2 S0 ; Disable Telnet
; Motors Part 2
G90 ; Send absolute coordinates...
M83 ; ...but relative extruder moves
M569 P0 S1 ; Drive 0 (X) goes forwards
M569 P1 S1 ; Drive 1 (Y) goes forwards
M569 P2 S1 ; Drive 2 (two Z motors in series) goes forwards
M569 P3 S1 ; Drive 3 (extr.) goes forwards
M569 P4 S1 ; Drive 4 (third Z motor) goes forwards
M350 X16 Y16 Z16 E16 I1 ; Configure microstepping with interpolation
M92 X80.00 Y80.00 Z400.00 E790 ; Set steps per mm (must come below M350 to include interpolation)
M201 X1000.00 Y1000.00 Z30.00 E200.00 ; Set accelerations (mm/s^2)
M203 X9000.00 Y9000.00 Z200.00 E900.00 ; Set maximum speeds (mm/min)
M566 X900.00 Y900.00 Z12.00 E120.00 ; Set maximum instantaneous speed changes (mm/min)
M906 X2000.00 Y2000.00 Z2000.00 E1000.00 I30 ; Set motor currents (mA) and motor idle factor in per cent
M84 S30 ; Set idle timeout
; Heaters
M307 H0 A125.5 C186.2 D2.7 V24.2 B0 ; Heater0 (Bed) ADC parameters (used by feedback loop)
M305 P0 T100000 B4138 R4700 ; Heater0 thermistor parameters
M143 H0 S120 ; Heater0 temperature limit (S120 = 120°C)
M307 H1 A473.5 C253.3 D4.4 V24.1 B0 ; Heater1 (Hotend) ADC parameters (used by feedback loop)
M305 P1 T100000 B4725 C7.06e-8 R4700 ; Heater1 thermistor parameters (for V6 hotend + Duet2Wifi: T100000 B4725 C7.06e-8 R4700)
M143 H1 S280 ; Set Heater1 temperature limit (S280 = 280°C)
; Fans
M106 P0 S0 I0 F500 H-1 ; Set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P1 S1 I0 F500 H T45 ; Set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on
; Tools
M563 P0 D0 H1 ; Define tool 0 (P0) w/ Extr.Drive 0 (D0) + Heater 1 (H1 -- should be "E0 Heater")
G10 P0 X0 Y0 Z0 ; Set tool 0 axis offsets
G10 P0 R0 S0 ; Set initial tool 0 active and standby temperatures to 0C
T0 ; Select tool 0 (anyway the only available for now)
; Automatic power saving
M911 S10 R11 P"M913 X0 Y0 G91 M83 G1 Z3 E-5 F1000" ; Set voltage thresholds and actions to run on power loss
) and is currently holding my spool -- so the 0.4 perimeters were fine, just the filling was messed up.