On Fri, 17 Jul 2020 09:32:04 -0700 (PDT), wrote:

I'm looking to cut 3 pass-through mortises (2"x6" long) on each of eight large pressure-treated square timbers (8"×8" wide x 8 feet long) for a small post & rail fencing project.
I've cut mortises on smaller thickness using just hand tools (drill, chise, wood file). I can't justify buying a plunger router or drill press just for this project and hardware companies near me don't rent them out. My plan is to purchase a longer drill bit, drill numerous holes along the inside edges, chisel out an opening on
each side, use a sawsall tool to cut around the perimeter, and use an oscillating tool to clean up the edges. This will take some time since I have 24 mortises to cut. Not needing a tight fit since the rails will be loosely placed inside the mortises. Any other suggestions on how to do this easily with hand power tools?

The thing that you want is some approximation of a chain mortiser.

Here's a guy who does it with big balls and lots of skill.
https://www.youtube.com/watch?v=wmWgtX9SE2E

Here's a guy with less skill who makes up for it with jigging
https://www.youtube.com/watch?v=h2CGslzGdB4

If you've by any chance got a radial arm saw this is worth a look:
https://woodworker.com/radial-saw-mortising-attachment-mssu-889-372.asp

If you're doing it with a chain saw, get an extra chain and sharpen
the teeth straight across instead of at an angle. I suspect any
electric chain saw will be adequate for this.