There's no need for combat. I gathered plenty of Chemical Manipulators in the Deciat system, from Unknown Signal Sources, specifically Combat Aftermath Detected ones. These are easily farmed.
Travel back and forth between the 1st and 2nd planets, scanning every USS you see. 2-4 will spawn for each one-way trip (of about 50 ls). Fly a little past the destination planet for the last USS to spawn. Sometimes I also shuttled between planet 1 and 4 or 2 and 4, depending on which was closer.
About 1/4 of those USSes will be a Combat Aftermath and about 1/3 of those will yield a Chemical Manipulator and occasionally, you'll get 2. Enter normal space if you see a Combat Aftermath. Use your left UI panel (3rd tab) to quickly identify the available materials. I typically gathered about 16 Chemical Manipulators in under 2 hours.
Arsenic is available on several moons in the Deciat system, 2 of them are near Farseer. I was able to scan wakes at the station closest to the main star and got quite a few Datamined whatever wakes. I engineered a 56 lyr FSD for my Asp (stripped and with a lot of engineered low-mass modules) even though I mostly stayed in the Deciat system.
Granted, I spent 8 weeks engineering, but I really wanted to go on the Distant Stars expedition. During that time, I gathered enough materials for about 250 level 5 engineering rolls for my FSD, mostly in the Deciat system.
Occasionally, in the Deciat system, an NPC would try to interdict me, so stay on your toes. Try to win the interdiction mini-game (don't submit), so that the pirates don't repeatedly try to interdict you.