So there was a new version of the RRF firmware (3.1.1-14) and that seems to fix the issue with the fan not starting properly. I also went back to Marlin for a while to get a comparinso benchy done:
IMG_20200719_141234.jpgIMG_20200719_141305.jpg

Quite ugly, not sure what's going on. Obviously cooling is an issue, but not sure if that causes the layer artifacts. Need to investigate more, but the main thing is that it happens on both Marlin and RRF so I can continue using RRF.
Easier prints that don't have layer problems:
IMG_20200719_141151.jpg
I'll continue debugging the quality issues in Dollo3D thread.

I tried the mesh bed leveling in RRF and it works quite well. I don't have a probe so the process is manual; you start the leveling process by issuing G29 after homing all axes. The web UI then shows a dialog where you can manually jog the Z height in 0.05mm incerements until it's good. After going through all the measurement points the results are saved to /sys/heightmap.csv. To use the saved file, issue G29 S1 in start gcode or before starting a print.
Compared to Marlin the process is quite similar, on Marlin I used the 12864 display and leveling menu to do it. Maybe bit more faster way, but usually there's no need to do the leveling process very often so not a big deal.
The nice thing about RRF is that the result file is a basic csv file on the sdcard which you can edit directly in the web UI or on your PC. On Marlin, you need to use gcode console to do the editing and the data is stored in EEPROM and can't be restored if you need to reset the EEPROM.

So far it seems that I'll be continuing to use RRF, since it seems to work quite well. Stability hasn't been an issue in use as long as I don't try to connect to web UI from multiple browers.
Pros:
- configuration stored in files on sdcard, easy to modify, easy to backup, easy to restore
- no need to compile FW
- general flexibility of the FW, about everything can be configured with the gcode
- web UI built in, no need for Octoprint
- ethernet/wifi support
- better driver timings it seems, printed movements seem 'cleaner', no resonating that happens on Marlin.

Cons:
- no backlash compensation support. Nice to have, especially with Dollo printer
- apparently no support for display on Sbase due to low memory. Not a big problem, web UI through phone/tablet works fine and does more
- experimental status of the FW. Seems stable but still