ZOA Center Controller reported they assigned a departing aircraft altitude below the Minimum IFR Altitude to avoid confliction with unidentified VFR traffic.

Narrative

Preface with this TRK departure and previous TRK departure both targets were not auto acquired which could be an issue with automation or TRK Tower controller. Usually departures are given a code and auto acquire 3 or so miles north of the airport but this aircraft and the one prior were told to squawk code. Traffic volume may have appeared normal however all morning there was an influx of IFR arrivals into TRK; TVL; MEV and CXP airports adding to workload. VFR flight following was also in high demand however I did begin limiting those requests due to traffic volume. During this situation I did have one IFR aircraft on a vector for approach into TVL. Aircraft X did not follow the MOWGL2 departure but instead appeared to depart and proceed direct to ZZZ. When I finally inquired the pilot assured me he was on the SID; at the same time the aircraft was climbing into a VFR aircraft within NCT airspace. Given the proximity and the fact that I was not in communication with the VFR aircraft I made a decision to stop the departure below the MEA/MIA (Minimum IFR Altitude) to avoid what in my opinion would have been an incident. I later asked if aircraft had terrain in sight meanwhile pilot was correcting back to the fix on the SID which was about 50 degrees left of his current position.I understand that there are better options in the moment but I did what was needed in my opinion to avoid an accident. Pilot later stated that he was having an issue with the autopilot. Sector 44 is a very complex and dangerous sector with high terrain and poor radar and radio coverage. It is situations as simple as this which can result in tragedy. I feel that NCT has better radar systems allowing for lower published MIA/MEAs and the ability to control departures and low altitude aircraft. NCT should take over TRK and TVL departures and arrivals due to the terrain and proximity to airspace / boundary.