Thanks for the quick reply. Firstly, small typo. It should be mtr, not mrt. I was confused at first.
No big deal, just in case someone else wants to replicate this.
As for the results, I will post four results in chronological order. It will become apparent that at times the connection works well, at other times it does not.
1st run: (240 probes, 1 second interval)
[xxx@xxxxx ~]$ sudo mtr -ezb4w -i 1.0 -c 240 8.8.8.8
Start: 2020-09-06T16:03:28+0200
HOST: xxxxx
Loss% Snt Last Avg Best Wrst StDev
1. AS??? _gateway (192.168.0.1) 0.0% 240 0.9 0.8 0.8 1.7 0.1
2. AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.254) 0.0% 240 8.2 10.3 6.9 61.0 7.8
3. AS3209 ip5886d7e6.static.kabel-deutschland.de (88.134.215.230) 0.0% 240 10.8 10.1 7.5 26.4 2.9
4. AS3209 ip5886c17e.static.kabel-deutschland.de (88.134.193.126) 0.0% 240 11.7 11.7 9.8 20.7 1.1
5. AS3209 145.254.3.94 0.0% 240 12.5 12.0 10.1 21.1 1.3
6. AS3209 145.254.2.217 0.0% 240 18.4 19.6 17.7 55.1 2.5
7. AS3209 145.254.2.217 0.0% 240 18.8 19.6 17.2 87.5 5.6
8. AS15169 72.14.194.138 0.0% 240 22.7 23.4 21.8 32.4 1.3
9. AS15169 216.239.62.103 0.0% 240 25.5 25.0 23.5 57.4 2.3
10. AS15169 209.85.251.131 0.0% 240 24.5 23.6 22.3 31.2 0.9
11. AS15169 dns.google (8.8.8.8) 0.0% 240 22.3 22.3 21.1 31.8 1.1
2nd run: (240 probes, 1 second interval)
[xxx@xxxxx ~]$ sudo mtr -ezb4w -i 1.0 -c 240 8.8.8.8
Start: 2020-09-06T16:08:02+0200
HOST: xxxxx Loss% Snt Last Avg Best Wrst StDev
1. AS??? _gateway (192.168.0.1) 0.0% 240 0.9 0.8 0.8 1.2 0.1
2. AS3209 ip5f5b6bfe.dynamic.kabel-deutschland.de (95.91.107.254) 0.0% 240 8.0 378.0 7.0 3065. 779.3
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
3. AS3209 ip5886d7e6.static.kabel-deutschland.de (88.134.215.230) 0.0% 240 9.6 394.5 7.5 3063. 797.9
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
4. AS3209 ip5886c17e.static.kabel-deutschland.de (88.134.193.126) 0.0% 240 10.9 397.8 10.4 3063. 802.5
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
5. AS3209 145.254.3.94 0.0% 240 12.1 393.5 10.2 3059. 790.0
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
6. AS3209 145.254.2.217 0.0% 240 19.2 398.3 18.0 3058. 782.4
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
7. AS3209 145.254.2.217 0.0% 240 19.0 373.6 17.3 2966. 750.4
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
8. AS15169 72.14.194.138 0.0% 240 24.2 354.8 21.9 2875. 716.8
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
9. AS15169 216.239.62.103 0.0% 240 24.6 334.4 23.0 2784. 684.3
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
10. AS15169 209.85.251.131 0.0% 240 24.3 352.4 22.1 3075. 716.4
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
11. AS15169 dns.google (8.8.8.8) 0.0% 240 21.8 381.4 20.8 3061. 757.4
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
3rd run (240 probes, 0.1 second interval)
[xxx@xxxxx ~]$ sudo mtr -ezb4w -i 0.1 -c 240 8.8.8.8
Start: 2020-09-06T16:12:52+0200
HOST: xxxxx Loss% Snt Last Avg Best Wrst StDev
1. AS??? _gateway (192.168.0.1) 87.5% 240 1.0 1.0 0.7 1.4 0.1
2. AS3209 ip5f5b6bfe.dynamic.kabel-deutschland.de (95.91.107.254) 80.4% 240 2633. 588.8 7.2 3051. 840.7
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
4th run (240 probes, 0.1 second interval)
[xxx@xxxxx ~]$ sudo mtr -ezb4w -i 0.1 -c 240 8.8.8.8
Start: 2020-09-06T16:13:57+0200
HOST: xxxxx Loss% Snt Last Avg Best Wrst StDev
1. AS??? _gateway (192.168.0.1) 87.5% 240 0.7 0.9 0.7 1.4 0.2
2. AS3209 ip5f5b6bfe.dynamic.kabel-deutschland.de (95.91.107.254) 84.2% 240 9.6 244.8 7.2 834.6 308.3
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
3. AS3209 ip5886d7e6.static.kabel-deutschland.de (88.134.215.230) 13.3% 240 8.2 79.5 7.0 928.9 191.2
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
4. AS3209 ip5886c17e.static.kabel-deutschland.de (88.134.193.126) 13.8% 240 12.2 78.3 10.1 919.6 189.1
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
5. AS3209 145.254.3.94 13.8% 240 10.9 77.1 10.5 910.1 186.3
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
6. AS3209 145.254.2.217 13.8% 240 19.4 83.9 17.6 903.5 182.3
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
7. AS3209 145.254.2.217 13.8% 240 19.3 82.0 17.5 894.0 179.7
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
8. AS15169 72.14.194.138 13.8% 240 24.1 84.3 21.8 886.7 176.9
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
9. AS15169 216.239.62.103 13.8% 240 24.5 84.6 23.1 878.6 174.0
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
10. AS15169 209.85.251.131 13.3% 240 24.0 85.1 22.2 867.9 176.5
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
11. AS15169 dns.google (8.8.8.8) 13.3% 240 22.4 82.4 20.9 858.0 174.2
AS3209 xxxxx.dynamic.kabel-deutschland.de (95.91.107.1xx)
So to me this looks as though the packet loss occurs right "after" the router, which is consistent with my previous observations. The question is: why? Again, if I connect directly to the cable modem, everything seems to be fine (tested for over 20 minutes at a time..).
Correction: there seems to be packet loss at the first step already. I overlooked that since latency seemed normal.
As for SQM: I currently do not have SQM installed/enabled. I wanted as little interference as possible. Right now everything you see is done with today's snapshot and almost no customization.