@Raniot Hello and welcome to the Duet world!
I'm guessing you have already read up on what ringing is and things you can do to change/improve it. In general it comes to trying to push a heavy print head round a corner too quickly, meaning the flex in the system bounces a bit and causes the artefacts you're seeing.
As well as changing the belt tension, you might want to double check all the joints/bolts/connections on the gantries are tightened - In the past I have found a screw loosened in the printhead which gave me some ringing.
After that you're mostly onto tuning the printer to try and slow it down round the corners. For this you can play with speed, acceleration, and jerk, and you can adjust them in your printer config, and/or in your slicer. General advice is to set sensible limits for the machine in your config file (i.e. things that will mean it won't skip steps, break itself etc), then set more detailed/controlled limits using your slicer. In the Slicer you can usually set different limits for different move types, so you can have fast, high acceleration moves forthe infil where ringing/quality is less important, but slower and more controlled moves on the external perimeters where quality is paramount. On an 800mm tall print like yours can do, this can make a world of difference on print times!
Regarding the firmware flavour, you should definitely set it to Reprap. Reprap and Marlin are very similar, but some of the commands are slightly different and use different units (e.g speed & jerk are in mm/min in Reprap but mm/s in Marlin).
I would start by using the values from your config file provided by the printer manufacturer in your slicer. I mostly use PrusaSlicer which only allows you to play with acceleration, so would recommend starting with just that, then you can play with Jerk later if needed. Looking at your config file, your machine has acceleration limits of 1000mm/s^2 in X & Y (from the M201 command), but these are then limited to 500mm/s^2 for printing moves (i.e. moves where the extruder is moving) and 1000mm/s^2 for travel moves (both from the M204 command). So I would put 500mm/s^2 for all the printing moves, and 1000mm/s^2 for travel moves in the Cura controls. Cura will then start inserting M204 lines in the Gcode files where needed to change these limits as needed during the print.
From there on, you mostly need to play with the slicer values on test prints until you get to a result you're happy with. At the same time, you might also consider changing the speeds for different move types in Cura (e.g. slowing down external perimeters a bit). For reference, on my big home made CoreXY, I print external perimeters at about 60% the speed of my infill, and with 30% the acceleration. You may be able to go faster than this, or might need to go slower - only tuning will tell!
Whilst tuning things, you could disable the acceleration control in the slicer and manually send different M204 commands during a print via the web control to see in real time what the results are. It may be a little harder, but will be quicker. There are a few good forum posts out there on the best methods for doing this, e.g.:
Regarding the slicer time estimations, you will probably find these get a lot better when you set up the accceleration control in Cura. I think the acceleration & jerk limits in your config are reasonably low compared to most printers (this is probably to be expected because your machine is so big, it will have a lot of mass to move around!), so will take a bit longer to accelerate/decelerate than Cura might expect by default.
If you're confused/unsure on the commands in the config file, you can always look them up here:
The descriptions are very good and will generally give you a good idea of what is going on, but it can be a bit daunting at first!
One final thought, you might want to consider using 'Dynamic Acceleration Adjustment' - M593:
This is a useful feature to combat ringing by tuning out the first natural frequency of your motion system. There's some details on what it is,how to use and tune it etc in the link above. First check if this is already in your config file from the printer manufacturer though!