The tip I received from the Pulp team was that Pulpcore 3.14 carried improvements for retrying downloads that should help. Not to mention that the Pulpcore from Katello 3.17 has the old tasking system which didn’t work so well.
Since you’ve already migrated partially to Pulp 3, I would recommend to continue upgrading up to Katello 4.1 so that you receive the best version of Pulp 3.
There’s an issue though. At the moment, upgrades to Katello 4.0 and 4.1 are broken due to the qpid-proton-c.x86_64 0:0.35.0-1.el7 package being in EPEL. More info: Katello installs for 4.0 and 4.1 are broken due to qpid-proton update in EPEL on EL7 - #45 by ekohl
I was able to work around it by removing the RPM, temporarily disabling EPEL, and installing the RPM from our koji build system: qpid-proton-0.34.0-2.el7 | Build Info | koji