Wiring through attic
Easy peasy.  This is what conduit is for.

I propose putting 2 circuits in it.  However, there are strict rules on conduit fill, and putting 2 cables in a conduit is impossible to do legally. So don't even try  - switch to THHN individual wires. Now you have plenty of room.  

Yes, you'll need to find an appropriate point to install a junction box.  Now, if you do any splicing in this junction box, any at all, you'll need to do the cubic inch math.  However if all wires simply pass through the box, and never will be spliced, then you can treat it same as a conduit body and any common box with enough physical space will suffice.  I'm a big fan of that one, so I advise running 2 separate ground wires - one for the range circuit and one for the garage circuit.  That way you don't have to splice in the box. 

If I wasn't splicing, I'd use a 4-11/16" square deep box.  This box must remain accessible, it can't be mudded or plywooded over.  

The oven circuit needs a #10 ground wire. It really should have been put in at the time, and you'll have a chance to fix that now. Its neutral also needs to be #8.  

I know you're familiar with 8/3 cable, but you cannot use 2 cables in a conduit - the only way it works is individual THHN wires.  You're not obliged to use conduit all the way - you can transition to 8/3 cable at any junction box with enough cubic inches to handle the splice.  But if you do run conduit all the way, you get to run a higher ampacity: #10=35A #8=50A #6=70A. 



The range needs a #10 ground (use bare to save conduit fill) and the garage circuit needs a #10 also.  

The #8/#8/#10/none wiring to the range was (well, is) incorrect.  It needs to be #8/#8/#8/#10.   Further, if somehow it is cable, it needs to be switched to THHN wire because of the conduit fill problem. 

Even if they are individual THHN wires, you need to pull them out of the conduit.  You cannot modify the piping with the wire in it. Code prohibits this, and you'll nick the wire - trust me.  Put the range circuit back with a #8 neutral and #10 ground.  I don't care if you convert your range connection to 4-wire (removing ground jumper) today, but I hope you do it soon. 

I've run the numbers and the biggest you can do is 
- a #8 circuit to the range and a #6 circuit to the garage
- #8 circuits to both range and garage, with space left over in the pipe for up to 2 additional circuits! 

A #6 cable to the garage could carry up to 70 amps if the whole run is THHN in conduit.  If you switch to cable for part of it, then you're limited to only 60A.   A #8 cable is similarly limited - 50A THHN-in-conduit, 40A if any cable is involved. 

Or you could go with a big subpanel!  You could potentially pull three #3 copper THHN wires (#8 ground) to the garage, and fit a subpanel there.  The #3 copper wire would allow a 100A subpanel.   That would fill the pipe, however you wouldn't have room for any more wires.  You would then have to double back from the subpanel to serve the range. 





* If you can't avoid a splice in a box, here are the gory details on the cubic inch math: Each #6 wire needs 5.0 cubic inches.  Each #8 needs 3.0.  Each #10 needs 2.5, #12 needs 2.25 cubic inches.  One #10 ground can be shared among all circuits. For cubic inches, you only count the largest ground once, and all other grounds are "free".