Not really, as far as I know the heavy cost is caused by the shaper itself, so it does not matter much whether you use HTB, HFSC, cake or ... So in short wondershaper is in all likelihood not much cheaper than sqm-scripts.
Great, I am interested to learn your opinion after that test.
Well, if only your brother and you are using the network each of you will get 50% of the bandwidth under full saturation load. Now there is going to be some serialization delay, but cake, AFAIK, will happily switch between users on a packet by packet basis and should be able to isolate your gaming reasonably well from your brother's you-tubing (assuming the 50% of the bandwidth is sufficient for each of you). But all theory is gray, so just go and test whether that works in practice.
Ah, in the upload direction, sqm does typically not need a big bandwidth sacrifice, with your VDSL2 link you should be able to run it at 100*64/65 = 98.46 % of sync rate (assuming your ISP does not also use a shaper/policer between the dslam/msan and the wider internet). But you will need to correctly describe the link characterization to sqm scripts (basically you need to define the correct per-packet-overhead). But in the download/ingress direction you will need to make a bandwidth sacrifice, otherwise any inrushing traffic will easily "spill-back" into the dslam's/msan's/upstream shaper's buffers and induce unwanted additional latency under load...
That does not seem to be possible...
Looking at https://www.scarlet.be/nl/internet/ the "loco" plan is 50/4, so 20/4 dos not seem to be sign of generosity of your ISP, but rather an indicator that your DSLAM/MSAN is slightly further away than ideal.
That seems like an over simplification at best; if training margin is the same as SNRM, then sure higher offers more resilience, but that actually is a value your ISP picks, in a reasonable noise free environment, 6dB is just fine. In your case since you are too far from the dslam/msan for full sync (50/4) I guess this really just shows your ISP configured its gear for >= 6dB SNRM.