We covered it in #3 & #5 and the OP thought the available space may be a problem?
Although if .5" Dia cyclinder were made from 1/2" pipe/tube and a 3/8"dia piston were used it should exert ~8Lbs force, a small air brush compressor is very low duty cycle but should have the capacity for a very short operation time.
Maybe a small automotive compressor used for air compensated suspension system are very small and 12vdc.
Max.

 

We covered it in #3 & #5 and the OP thought the available space may be a problem?
Although if .5" Dia cyclinder were made from 1/2" pipe/tube and a 3/8"dia piston were used it should exert ~8Lbs force, a small air brush compressor is very low duty cycle but should have the capacity for a very short operation time.
Maybe a small automotive compressor used for air compensated suspension system are very small and 12vdc.
Max.

There's also the option of going hydraulic... but maybe cost is an issue here.

 

There's also the option of going hydraulic... but maybe cost is an issue here.

+1
Hydraulic doesn't have to be expensive. This little $6 cylinder has a .55" diameter.

Wouldn't even necessarily need a traditional HPU. Automotive parts could be repurposed to achieve the same concept as I mentioned before (remote power, mechanically transmitted).
A clutch or brake master cylinder could be electrically actuated to force fluid down to the mini-cylinder in the same manner as your foot does.

 

Oh, and I forgot to highlight the fact that, from a hydraulic standpoint, the master/slave cylinder concept has the added bonus of not requiring any solenoid valves.

By my math, that little cylinder I linked to would require 41PSI to generate 5lbf. no big deal.

 

A automotive brake cycl is one way acting so somehow the slave cylinder would need a method of return, possibly a spring of some kind, but this would add to the torque requirement.
But may still be doable.
Max.

 

The torque required can be calculated, a rotated lead screw is a class 1 lever.
Max.

What I was getting at was that at some point increasing the pitch the "thread" becomes more like a spline and I'm guessing that that point is somewhere close to an imaginary 45 degree "thread slope" - i.e. if you project the thread on to a flat plane it would form a 45 degree line (in practice, unless you have a ball lead screw, I would expect friction to be the limiting factor long before 45 degrees). So by doing that maths I meant something like diameter/pitch

 

If using the class one lever example the dia is the input, and decreasing the pitch, decreases the output, or increases the advantage.
Max.

 

If using the class one lever example the dia is the input, and decreasing the pitch, decreases the output, or increases the advantage.
Max.

Use a ball-nut/screw and efficiency would be maximized, Max (pun intended)