Electrical – Should I extend existing 12/3 circuits with 12/2 for wiring attic

What you have here is a line-side traveller loop. The white wire in the leftmost 14/3 cable takes power out to the switch in Box #2, while the red and black wires in that cable are the travellers going back to the switch in Box #1, which then feeds the load.

The fortunate thing about this configuration is that it's relatively easy to hook your smart switches up to it. You'll just need a couple of pigtails for the line and neutral on your new dimmer, and a couple of wirenuts to cap off the unused black wire in the cable between the two switch boxes.

1. Make sure the power's off at the breaker.
2. Remove the old switches.

In the first box:

3. Remove the white wire that's bundled in with the black wires from that junction and join it and the white pigtail to the existing bundle of white wires. (These are your neutrals.) Remove any tape or paint flags or markings on the white wire in the leftmost cable. It will become a neutral once again, as it was a hot previously.
4. Nut the black pigtail into the existing bundle of black wires. (This is your hot feed coming into the box.)
5. Connect the existing ground pigtail that grounded the old switch to the ground terminal on the smart switch if the old switch was grounded. If it was not grounded, post a comment here and I'll update the instructions.
6. Connect the black pigtail to the line terminal on the smart switch.
7. Connect the white pigtail to the neutral terminal on the smart switch.
8. Connect the red wire from the leftmost cable to the traveller terminal on the smart switch.
9. Cap off the black wire from the leftmost cable with a wirenut.
10. Connect the red wire from the center cable to the load terminal on the smart switch.

In the second box:

11. Remove any tape flags or paint markings present from the white wire.
12. Cap off the black wire with a wirenut.
13. Connect the ground wire (if present) to the remote's ground terminal. If no ground wire is present, post a comment here.
14. Connect the red wire to the remote's traveller terminal.

15. Connect the white wire to the remote's neutral terminal.

16. Put everything back together in both boxes.

17. Turn the power back on and configure the smart dimmer.

So let's consider just the junction box for a second

There's 3 wires intersecting here

1. The hot line from a prior outlet/circuit breaker
2. The line to your outlet
3. The line to your attic light

There's room for a fourth wire here so that's what I would suggest. But you'll need to make another store run to do it. Return your 25' 14/2. My local big box sells 15', so if you can buy some 15' 14/2 do it. Then get 25' of 14/3. Now, let's get your light in (be sure to turn the power off first!)

1. Cut the hole for your fixture and mount the box above it. If I were you, I'd go the extra mile here and mount a box attached to a bracket, just in case you ever want to hang anything significant off it.
2. Put your 14/2 into the junction box in the picture above (you have an open clamp on the left side). Just attach the wires to the existing nuts as they already are (white to white, etc). Run the other end into your new fixture junction box
3. Cut a hole for your old work box in the drywall for the switch. Don't mount the box in just yet, just make sure it fits for now and then leave it out
4. Near where the 14/2 for the outlet goes into the wall, drill another hole (needs to be at least 1/2" or greater). The tricky part is you need to make sure you're in the same cavity as the hole you just cut in step 3. Once you have the hole, fish your 14/3 down here. You'll want a helper down by the hole who can grab the wire and pull it through
5. Before you go down, put your 14/3 into the box for your fixture (can cut it now or fish and cut below)
6. Inside the fixture box you'll attach the wires. White to white and bare to bare, but attach the red wire to the black wire in your 14/2 (nut this one off but leave the rest un-nutted). Leave the black from the 14/3 by itself for now.
7. Put your 14/3 into your switch box and mount it. When you put the switch in, attach the red and black (doesn't matter the order) and your ground. Strip the white and just put a pigtail on it. Mount the switch.
8. Mount your fixture. White to white and black to the 14/3 black that's by itself. Put nuts on the rest of the wires now. Don't forget to ground it!

You can turn the power back on now and your fixture should work. Why the extra white wire in the switch box? Code now requires a neutral in the box. Some modern switches need one (like wi-fi switches). The cost here isn't that much more, but it will save you a lot of hassle if you do it right now.

There's one thing I did want to note here before closing. Your outlet is a 20A outlet, but your wire is 14/2, which is 15A max. That's not safe. I would buy a 15A outlet and replace it.