yes, there are lots of ways to optimize.

23G sounds good for drawing, and I think that is the more standard recommendation (looks like the /r/TransWiki guide lists 23G and 25G), though I haven't used it personally.

Re-using the same needle you drew with to inject with can make the needle a bit more dull and I have found it makes the injection more likely to hurt (apparently I'm not the only one). It is also standard practice to change the needle between drawing and injecting to further reduce chance of infection. I've read that plenty of people don't change needles between drawing and injecting, but I'm not sure I would necessarily advertise this as a good practice, esp. for beginners.

Part of the reason I suggested subq is because it allows for injecting with a smaller gauge needle like 27G, which is less painful and more accessible to people like me who suffer from needle phobia. That's part of why I think of it as more foolproof, but also because subq doesn't require targeting a specific muscle or injecting to the correct depth like IM does.

I'm not sure about irritation from subq that IM doesn't cause, so I would love more detail on that. I just would imagine the smaller needle, the shorter length, etc. makes it less irritating than a larger gauge and longer needle.

You can also buy low deadspace syringes without using a fixed needle, but I find these kinds of optimization less relevant when regularly accruing excess medication and there is incentive to use up old medication before it expires. A vial is like $11 for me, so it is also rather cheap, and the medication waste in that context is not worth overly concerning yourself over.

It would make more sense for DIY, though, when the medication is more expensive (and supply might be less reliable and more scarce). I have never looked at the extra cost of low deadspace syringes compared to the cost of wasted medication from that deadspace, but even with normal syringes I know people have used bubbles in their doses to leave air in the deadspace and get all the medication (correct dosing becomes a bit tricky in this situation, just have to be mindful that you are measuring your dose accurately).