Best way to trim a door lengthways

I hate working with doors as I always get the gaps wrong or shave too much off or unevenly, but I'm not a clumsy persons. I just can't seem to get it right.

I don't like using a power plane as it can dig out too much or do the wavy thing or go downhill, and a hand plane takes forever. My preference is to use a small power saw on a ply guide as I can guarantee getting it to the line across the length, but as I discovered yesterday, trying to remove 3mm with a power saw, it would take off the top but not the bottom, so I had to do three passes pressing firmly on the base and then square it with a hand plane. Tedious.

Any other ideas (apart from that Makita guided saw)?

Hand plane.

1. It must be sharp.
2. Have a way to hold the door solidly for edge planing, not just between your knees.
3. Two planes are better than one. A #5 with a cambered blade to hog off the majority of the waste, followed by a longer plane to put a nice finish on the square edge.

Here's and example..
https://www.youtube.com/watch?v=Mvmk4Jc0bCo

Not sure why you had the problem using the circular saw. :? Have you checked the angle of the blade to the sole of the saw is 90 degrees? Is there something wrong with your setup that caused the blade to cut an angle greater than 90 degrees? I've used your method many times for adjusting the width and length of doors and never had this problem.

I must say his hand planes work better then mine. I have a simple stanley block plane that never seems to work and I have the identical longer plane that I used on the job, but I didn't have my sharpening stone with me so it was less than ideal.

I'm going to check it out on some waste timber and play around with an old door to work out what the issue is. I think the blade or plate flexes or I may need to set a negative angle of 1-2 degrees.

There could be any number of reasons why you would want to trim a door. The most common one being a sticky door. In this case, Trimming the door should always be the last resort and most of the time not necessary if the door was hung properly in the first instance. For existing door that is sticking to the jamb, First you should remove the hinge pins and see if the door fits into the jamb nicely. if it does, then your problem can be fixed by adjusting the barrels of the hinges, you can do that by using a shifter to move the barrel on the jamb side to line up with the barrel on the door side. Put the hinge pin back, test and re-adjust if necessary.
If the door is just too big to fit into the jamb even after removing the hinge pins, then trimming is needed. Most of the time, we are usually talking about half to one 1mm here and there, sometimes as a result of the jamb going slightly out of square. In this situation, I'd advise against using a circular saw. Using a hand plane, probably a #4 is the best option. Block plane isn't much good as you are using it with one hand. The blade needs to be really sharp. If you don't have a hand plane with a sharp blade, you could at the pinch using a surform for that purpose.
If the amount to be removed is more than 1 mm on an old door, then you should really check the jamb for plumb and square because something has moved quite a bit. I had that happened on my laundry door and it turned out that the jamb has detached from the brick wall and once I fixed that, the door closes. If everything checks out, then I guess you'll get to use your circular saw.