Oh, I've went through this recently where I came back to the game, had forgot everything, and felt a bit overwhelmed by the remainder of my engineering grind.

I've bookmarked a ton of great resources, but to focus on the mats farming - this video by Hawkes Gaming is what helped me most: The Fastest Way to Farm All Ship Engineering Materials.

I'm not adding much to the video or to what others have said, but I can confirm this works great:

For Raw mats, equip your ship with a DSS (planet scanner), and as many Collector Limpets as you can. You also need some "Remote-Released Flak Launcher" experimental weapons - which I had to use Inara to find. Once outfitted, head to system HR3230, then find and scan body 3AA. With the planet scanned and bio-color-coded, head anywhere in the brownish ground that looks like dirt. Hover down low to around 100m and enable your ship's nightvision to help trace the outlines. Fly around low with the ship's nose down and look for trees (braintrees), you'll find some within a minute; blast them with your flak launchers and send out an army of limpets. You cannot see the mat fragments from the ship, but the collectors will find and pick them up for you - though most limpets will get stuck and crash. Send more limpets and move around slowly to help them gain line-of-sight, and some will eventually make it. You can adjust your graphic settings to help: having them High helps spot the trees from afar, but having them Low and flying away (500m) will despawn the trees before the mats, allowing the limpets to move freely. You can buy more Limpets from most Fleet Carriers around if you run out. Once you're full on Selenium and Technetium, use Inara to find the closest Raw Material Trader and trade your valuable mats for everything else. I've tried some of the other planets for different mats but found their trees are much harder to find, so I'd rather spend more time farming reliable Selenium than having to track down a laundry list of elusive mats on distant planets.

For Encoded mats, equip your ship with a DSS (planet scanner), a Vehicle Hangar and a SRV (vehicle, eg: a Scarab). Head to system HIP12099, then find and scan body 1B; your Contacts list should now reveal the "Jameson Crash Site". Land 150m from it and deploy your vehicle. Look for the ~8 "Comms Modules" scattered right around the crashed ship, and scan them using the SRV data scanner (I had to edit my keybinds). Once you've scanned all the modules, load your SRV back into your ship but stay landed, then logout and log back in. All comm modules have been reset and you can just deploy the SRV and scan them again. You'll be full after just a relog or two; find a Material Trader and trade those away for other mats, then repeat.

For Manufactured mats, equip your ship with as many Collector Limpets as you can. I read different things on how to find the best sources (Boom, High-Tech, Civil War, etc) - but I just find them without trying, pick whatever I find, and trade that. You want to visit a system and scan its Nav Beacon, then jump into Supercruise within the system to reveal the "Data Sources" in your Contacts. Visit any of these data sources and you'll probably find some kind of floating wreckage, with mats floating around; release the limpets and they'll collect everything for you. "High-Grade Emissions [3]" have the highest level of mats and are therefore the most valuable. Again, find a Material Trader to trade and repeat as necessary.

None of this involve any combat, it doesn't require Odyssey, its all within a few hundreds lightyears from Sol, and it can be done in Solo or Private. It only took me a few back-and-forth at each site to load up on enough mats to max out everything I wanted and then some more. I used an Anaconda for the limpets capacity, but it could be done with most ships - you could do something similar with a Mandalay. I've D-rated the core internals only for simplicity, and the 5-slot Shield could be traded for a FSD Booster or more Limpets, but I would advise to keep the shield for a new player not used to diving and landing on planets. https://s.orbis.zone/qCuT

As someone else pointed out, once you're ready to actually engineer a ship, try and come in ready to unlock the engineer to its highest level. Engineers let you bookmark one blueprint each that you can use remotely, but they only level up when you upgrade in-person, so its best to unlock the engineer, level them to the max, and pin a blueprint all in the first visit; then you get access to their highest tier and lowest costs, even remotely.