Success! Adding 2 x 8GB RAM to my TS-451+. TrueNAS is running way better with the additional RAM. 8GB was enough just for basic file sharing but it did run into issues
TS-451+ - Features | QNAP
Parts-Quick 4GB Memory for QNAP TS-451D2 Compatible DDR4-2666 260 PIN SODIMM RAM at Amazon.com
QNAP NAS Memory Upgrade Guide - Quick tutorial on increasing your RAM
TS-451+ | Hardware Specs | QNAP (US)
How to Upgrade the Memory(RAM) of NAS | Qnap Advanced Support