The part that is missing from your question is, in a parallel reality where you didn't buy the PS5, how did you spend the extra money? Also, how did you get that money in the first place? Maybe the parallel reality we should look at is the one where you worked less, got less money, and therefore didn't buy the PS5. Or maybe you worked equally hard, but was less successful at negotiating salary, so your boss bought one more PS5 for their kids instead.

I think the standard assumption is that the entire circle of "you do something useful to get money, then spend the money to buy PS5" is generating something positive for both you and the others. -- And the "magic of capitalism" is that this is actually not obvious from the perspective of entire human history, because some traditional alternatives are "you extort money from people at a gunpoint, then spend the money to buy PS5", which do not generate so much positive value for the others.

Now a separate question is whether it would be better for others if instead of buying the PS5, you donated the money to an effective charity. Yes, it would be, obviously. But how many people actually do that? -- Here again, the "magic of capitalism" is that people generate good as a side effect of their selfish actions, which makes it way more reliable. (And this does not prevent anyone from creating even more good by donating.)

tl;dr -- (under some theoretical assumptions which are not necessarily true in the real world) participating in normal trade generates some good as a side effect, and of course donating to charity instead would generate even more good

there's a negative externality of using up some of civilization's finite productive capacity.

That's a very unusual and IMO confusing framing of things.  "negative externality" normally means things that impinge on others' rights - pollution from making the PS5, for instance.  "civilization's finite production capacity" isn't actually anyone's right, so it can't be impinged or considered an externality.

Insanity Wolf would tell you you're absolutely right.

NEW PS5?
HOW MANY DEAD BABIES DID IT COST?

DOING GOOD ISN'T THE MOST IMPORTANT THING
IT'S THE ONLY THING!

PERFECTION IS COMPULSORY
ALL ELSE IS FORBIDDEN

THERE IS NO END
TO DOING GOOD

RESTING?
REVEALED PREFERENCE!
YOU DON'T CARE ABOUT THE SUFFERING!

YOU MUST DO THE VERY BEST THAT YOU CAN ALL THE TIME, WITHOUT END
IT’S A THEOREM!
YOU CAN'T ARGUE WITH A THEOREM!

And so on.

However, "negative externalities" normally means things like pollution from mines that the mine-owner does not have to pay for, not merely that someone is making game consoles instead of medical equipment. Positive injuries caused by one's positive actions, not merely a failure to devote all one's resources to curing the pains of the world.

It all depends on where you draw the baseline: inaction, or perfection. Is the question, am I making people worse off than they would be if I had done nothing? Or is it, am I leaving people less well-off than I might have made them if I had acted as well as possible? There are those who argue the latter, such as Peter Singer, but I always hear underneath the words the shriek of a rabid wolf, and turn my back.

Conspicuous consumption luxury spending is not a good use of resources and is a negative externality. Some spending is very beneficial such as that on basic research or demand for healthy goods which leads to more production of that healthy good. Spending money on something signals that their is demand for that good or service so leads to more production of that good or time spent on that service. Society would be better off with more medical research and less yachts and JPEG NFT's so we want to encourage spending on health instead of some luxury goods.