So I concluded that my low percentage was due to the fact that I was cryptomining using CPU on the same computer (although using only 3 cores out of 4, i.e. 3 threads out of 8).
By stopping the CPU mining, I was able to raise upload success rate to over 40% in 3 days as well as repair to over 40%.
To give a better idea for the node:
CPU: Intel Core i7-4700HQ 4x2.4 GHz (4core/8thread), boost up to 3.4 GHz
RAM: 2x8 GB DDR3 1866 MHz
System drive: Kingston SSD 240 GB SATA3
Node drive: Seagate Constellation ES.2 3 TB HDD
OS: Ubuntu 20.04 LTS
Network download/upload: fiber-optic 1Gbps/600Mbps
Also, the traffic has started coming in since yesterday even more than in May:
And the successrate:
========== AUDIT ==============
Critically failed: 0
Critical Fail Rate: 0,000%
Recoverable failed: 2
Recoverable Fail Rate: 0,551%
Successful: 361
Success Rate: 99,449%
========== DOWNLOAD ===========
Failed: 14
Fail Rate: 0,276%
Canceled: 5
Cancel Rate: 0,099%
Successful: 5052
Success Rate: 99,625%
========== UPLOAD =============
Rejected: 0
Acceptance Rate: 100,000%
---------- accepted -----------
Failed: 0
Fail Rate: 0,000%
Canceled: 35134
Cancel Rate: 47,822%
Successful: 38335
Success Rate: 52,178%
========== REPAIR DOWNLOAD ====
Failed: 0
Fail Rate: 0.000%
Canceled: 0
Cancel Rate: 0.000%
Successful: 0
Success Rate: 0.000%
========== REPAIR UPLOAD ======
Failed: 0
Fail Rate: 0,000%
Canceled: 3377
Cancel Rate: 57,500%
Successful: 2496
Success Rate: 42,500%
========== DELETE =============
Failed: 0
Fail Rate: 0,000%
Successful: 1703
Success Rate: 100,000%