I've found that almost everything I've needed for the few Engineer mods I've gone for, has come to me 'in passing' while doing other things.
Wake Scans for example. You can either sit outside a station for a long time doing nothing but trying to scan every wake, or just scan any wakes that are convenient as you're coming and going about your usual business.
Data from ship scans. Just target any ships that happen to be in front of you as you're flying in SC or around stations.
Natural materials can be picked up while doing an occasional bit of mining, usually for missions, or collected in passing while on surface missions.
Kill Skimmer missions which have you driving around POIs close to settlements are also a good incidental source of materials and scans (from crashed ships, rocks you're driving past, and the like), without taking you out of your way.
Dropping into convenient Unknown Signal Sources can net you materials, cargo, scans and/or easy bounties without undue effort.
Not to mention all the goodies you will naturally accumulate from mission rewards.
Doing this sort of casual scavenging while otherwise playing the game as you want will get you most, if not all, of what you need. Of course, if you feel a burning desire to have a specific mod and
need it now, you'll probably have to do the occasional bit of focused searching for materials/commodities, but that just adds variety for me.
Incidentally, if you need to do the 'travel X distance to unlock' then combining it with a Passenger Transport mission can net you a
very healthy profit at the same time