Is it worth insulating a return duct that runs through my attic?

Your instinct is right, that duct is absolutely picking up heat in the summer. Even though the air inside is close to room temp, a bare metal trunk in a 130 degree attic radiates straight thru the sheet metal, and every degree it picks up is cooling you paid for. I did this exact job on my own return a couple summers ago and coud feel the difference at the registers on hot afternoons afterward.

First thing I'd do before any insulation goes on is seal the seams and joints with mastic. A return runs under negative pressure, so any gap in that trunk is actively sucking attic air, with all its dust and loose insulation fibers, right into your system. Get a bucket of mastic paste and brush it over every joint, every screw hole, every corner. Let it cure fully before you cover anything up, since you won't get another look at it once it's wrapped.

For the insulation itself, I went with foil-faced fiberglass duct wrap, the kind made for this exact purpose. Wrap it with the foil facing out, overlap the seams, and tape every joint with foil HVAC tape so it's actually sealed, not just draped over. The vapor barrier facing matters in summer because a cold-ish duct in hot humid attic air will condense on the outside if air can reach the metal. The rigid foam board jacket approach works too and gives you a slightly better R-value per inch, especally if you build it like a little tunnel over the trunk and seal the corners with canned foam or tape. Just keep the foam off direct contact with any hot equipment or flue nearby, foam doesn't love high heat.

Either way, don't skip the mastic step. The wrap slows heat gain but it does nothing about air actually leaking in through the seams, and that's the part that pulls attic junk into your air handler.

One thing to check before you wrap, since I learned this the annoying way on my own house: make sure that trunk is actual duct the whole run and not a joist bay or some framed cavity being used as a return. Mine looked like metal for most of its length but the last section was just framing with sheet metal nailed across it, and it was leaking like a sieve into the attic even after I sealed the visible joints. Also, where the duct sits up on the framing, the straps or blocks touching it are little heat bridges, so I tried to keep the wrap continuous under it rather than stopping at each hanger. Made the job fussier but the difference in the summer bill was real.

Not buying the condensation warning in that second-to-last paragraph. A return duct is carrying house-temperature air, so in summer the metal is never colder than the attic around it, wich means there's no dew point problem on the outside of the wrap. The foil-out advice is fine for other reasons, keeping radiant heat off it mostly, but I think the fogging concern got mixed in from supply duct thinking, where you actually do have cold air running through the thing. When I wrapped my own trunk I just used the foil side out anyway since that's how the roll comes, but I didn't stress about perfect sealing of the facing. Overlapped joints taped down good was plenty. If anything I'd argue effort is better spent on the seams underneath than fussing over the outer barrier.

Seconding the poster who said to check wheather the whole run is real duct. The way I found mine was dust streaks on the metal, like faint dirty shadows right at the seams, which meant attic air was getting pulled through the gaps every time the blower ran. I opened up the insulation expecting to patch a couple joints and ended up rebuilding the last ten feet with proper sheet metal and screws because half of it was just a panned bay. whould've been a waste to wrap over that.

Following this one closely since my setup is similar but worse for access. My trunk runs right up against the roof sheathing in spots, so there's barely a few inches of clearance to work with. If I wrap it with anything decent thickness, I'm worried the top side ends up squished against the plywood and half useless, or I can't even get tape pressed on properly up there. Did any of you deal with tight spots like that? Curious if it's better to do a thinner layer over the whole run or leave the crushed sections alone. Also not sure if the duct hangers and straps need wrapping too or if leaving those bare metal defeats the whole thing.

Update: took the advice about checking the run first and I'm glad I did. Found faint dust streaking at a couple seams near the far end, so I masticed those before wrapping anything. Turned out to be real duct the whole way, just leaky. Foil-faced fiberglass wrap is on now, taped tight, and the upstairs rooms stopped feeling stuffy by afternoon. Appreciate the push to inspect before insulating.