Here's one thing I see:
00:00:00.171911 [/Devices/vga/0/Config/] (level 4)
00:00:00.171915 3DEnabled <integer> = 0x0000000000000001 (1)
....
00:00:00.171975 VMSVGA3dEnabled <integer> = 0x0000000000000001 (1)
00:00:00.171979 VMSVGAEnabled <integer> = 0x0000000000000001 (1)
00:00:00.171981 VMSVGAPciBarLayout <integer> = 0x0000000000000001 (1)
00:00:00.171983 VMSVGAPciId <integer> = 0x0000000000000001 (1)
00:00:00.171986 VRamSize <integer> = 0x0000000004000000 (67 108 864, 64 MB)
Correct VMSVGA video card for Linux guests, but probably way too little video memory for 3D acceleration. Max that slider out, 256MB.
There's something unusual about the Guest Additions:
00:00:15.607500 VMMDev: Guest Additions information report: Version 6.0.6 r129722 '6.0.6_KernelUbuntu'
00:00:24.087251 VMMDev: Guest Log: 00:00:00.000793 main Executable: /opt/VBoxGuestAdditions-5.2.18/sbin/VBoxService
Uninstall whatever Guest Additions are in the guest then install the Guest Additions from the built-in ISO that came with your 6.1.0 Virtualbox (guest Devices menu, Insert Guest Additions CD image, look in guest's CD drive). Matching Guest Additions is probably necessary when running with 3D acceleration.
00:00:00.400793 CPUM: Physical host cores: 16
Sweet PC! Please pack up and send it to me so I can test this issue further.