Thought I should update this.

I changed my plans slightly and instead of uploading Chrome I managed to modify my stock ECU software with BlackStealth's help. The reflash capabilities available for the '99 Euro ECU are much more limited than those available for Chrome, but enough for my application.

With EcuFlash I could change injector size and voltage latencies, target AFR and timing by engine load x rpm as well as "boost limit".
I had 550cc Bosch EV14 injectors installed, they came from a local injector specialist as a flow-matched set of 6 injectors with flows and latencies measured on a flow bench. I simply plugged the flow values and latencies provided on the injector test sheet into EcuFlash, reflashed the modified stock software via Openport 2.0 and it idles and drives smoother than stock

Unlike Chrome, the stock ECU does not allow for controlling boost. I am using BlackStealth's OLEDBC (LCDBC with an OLED screen) instead, which comes with so many handy features that it would be useful to have even if I had ECU-controlled boost. I'm still troubleshooting a boost overshoot issue - likely either normal full-throttle boost spike (wastegate lag) or a leaking wastegate vacuum hose - but otherwise everything seems to work fine.

The "boost limit" parameter in the ECU limits the maximum engine load value allowed by the ECU before it cuts the fuel. When I was testing and tuning the OLEDBC settings on the road, I briefly got it to work and going full throttle was like being rear-ended by a freight train... and then hitting a brick wall. For a split second I was sure I'd broken something... until I remembered that BlackStealth had advised me to increase the boost limit to the highest available value. Fuel cut during full throttle acceleration feels surprisingly violent.

The last one could cause the issue jannoke described in the previous post.

I haven't yet modified the AFR and timing tables, but will tune for a leaner AFR once I get the boost issue sorted. There's a general rule to tune for 11.5 AFR at max boost, tapering to 11.0 by rev limit which should be doable based on datalogging on the road. I'm not aware of a similar rule of thumb for timing, and the forum wisdom of "tuning until knock" seems somewhat risky to an amateur. Might leave the timing map untouched for now and have a specialist tune it on a dyno once I throw in larger turbos and intercoolers.

Hope someone finds my amateurish experimenting useful. Many thanks again to BlackStealth for helping out!