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 a debian distro, and put it to /lib folder on the NAS
https://packages.debian.org/stretch/armhf/libstdc++6

cd /tmp
wget http://ftp.us.debian.org/debian/pool/main/g/gcc-6/libstdc++6_6.3.0-18+deb9u1_armhf.deb
dpkg -x libstdc++6_6.3.0-18+deb9u1_armhf.deb libstd
sudo cp libstd/usr/lib/arm-linux-gnueabihf/libstdc++.so.6.0.22 /lib
cd /lib/
sudo chmod 555 libstdc++.so.6.0.22
sudo ln -f -s libstdc++.so.6.0.22 libstdc++.so.6

status pkgctl-jackett-dotnet
start pkgctl-jackett-dotnet
tail /var/packages/jackett-dotnet/target/var/jackett-dotnet.log