I very quickly ended up on 8-10 jump journeys [...] My Type-9 is shielded and unengineered; do I need to be maximising jump range and/or cargo capacity
Yes, you clearly need more jump range. Get a long range 6A FSD and a class 4 or 5 FSD booster
ASAP. Minimizing weight, and Mass Manager on the FSD, are obviously beneficial, but
far less impactful.
My thinking wrt. cargo capacity is: putting cargo racks in the top 4 slots (8,8,7,6) gives you 704t of capacity. Theoretical maximum is 790, so if you prefer doing 11 runs in safety and comfort instead of 10 on a bare-bones ship, there's enough slots left to install both flight assists, a guardian-boosted shield, a rescue limpet controller, an SRV bay and a sauna. Or whatever. Just remember to fit the FSD booster, otherwise you'll be doing a
slow comfortable trading trip.
For reference:
My T9 has 29Ly range with 752t cargo, 42Ly without.
Class 5 FSD booster, hence shieldless, hence 7A dirty thrusters.
I traded (mostly late Friday and Saturday):
A) Insulating Membranes from Bridger Terminal in HR 827 for 24MCr per 3+4 jump trip.
Probably too far, but had consistent supply, so I didn't bother searching for something closer.
B) HN Shock mounts from LFT 37 for 4MCr per 1+1 jump trip.
The system as a whole had no supply shortage either, although I needed to switch stations twice in the ~10 trips.
I was quite surprised when I found this (and that it didn't dry up), but apparently most people were trading the (far better paying) Membranes/Beryllium at the time.
What mistake am I making here? Is this a simple case of "don't expect top tier success without a Fleet Carrier"? Is it a first come, first served thing, and my mistake was not getting more done on day 1? Do I need to be more strategic with where I'm buying my cargo - aim for the "not quite nearest" to buy a little time while the nearer stations are depleted? Since the contributions are ton-based rather than credit-based, should I be focusing on the cheaper options because they might not sell out as quickly as the better profit ones?
Not an experienced trader, but yes, I'd rather do the extra jump than arrive at a likely-empty station. See A) above.
And you need to know what your goal is and trade accordingly. If you're trading for participation percentile, then cheap/abundant goods close by is indeed the game.
Lastly, if you can't find attractive targets, don't forget planetary stations and stations far away from the jump point. Sure, it's extra effort, but maybe it's
less extra effort than a conveniently-located station in a system several jumps out.