SPK for Synology using dot-net framework rather than Mono for arm7,arm x64 and x86 is available here:
https://github.com/publicarray/spksrc/releases/tag/jackett-dotnet-0.16.1583_1

Manual update when auto-update fails:

cd /var/packages/jackett-dotnet/target/share/
sudo mv jackett jackett-old
sudo mv /tmp/JackettUpdate-v0.17.*/Jackett/ jackett

Update 0.16→0.17 fails on synology due to dotnet 5 dependency issue (libstdc++ 6.0.22):

    copy libstdc++.so.6.0.22 from my debian distro, and put it to /lib folder in DSM
    chmod 555 libstdc++.so.6.0.22
    change symbolic link libstdc++.so.6 to this file (ln -f -s libstdc++.so.6.0.22 libstdc++.so.6)
    run dotnet --info
    It run without the error.