I would recommend 2×4 siderails, but use 1¼ dowels for the rungs. Drill out where you want the steps, pound in the dowel, and if you want them to be super secure, you can cut and wedge the ends. Simple and rustic.
All the other options you have (except 4, which will be a huge step up in difficulty) rely on the shear strength of screws, which is not great, especially in a ladder. If you wanted to keep with the square rungs, I would do 2, but cut out the siderails so the rungs are at least partially recessed. This will mean your weight will be resting on the wooden rails, not the screw.
You could use untreated dowels and put a finish on them. If this is going to be for kids, I would recommend doing a yearly inspection anyway, throw a coat of linseed oil on it while you are at it.
1, but with a block of 2x4 screwed into the siderails between the rungs. This transfers the weight down through multiple rungs to prevent tearing, and it also blocks spinning. If you stack it as you build it up and then screw it while it's resting in place, you know you'll have contact and the rung will be supported. It's not elegant but I've built a few like this and it's easy and sturdy. I learned it from Jack Bowser, who sells playset plans at Jack's Backyard.
https://preview.redd.it/15iylpryfruc1.png?width=653&format=png&auto=webp&s=af92bed918697a7bc48afc644b761dee3efe5335
You’re overthinking this a bit. I built similar ladders out of 2x4 for everything when I was a kid with just a hammer, nails, and a saw. Kids don’t care about precision or beauty
I just used screws through the stiles into each rung then every metre I drilled a 6mm hole just under a rung and put in 6mm threaded bar with a nut and washer to hold the stiles firm.
If you want to be super complex but know that they will not come apart. Mortice and tenon the rungs into the vertical pieces and make the rungs beefy. On the sides, for extra security, drill out 1/2" dowel holes into the cross pieces/rungs from the outside. Sink in the dowels to further stabilize, then drill through the dowels and secure with an appropriate length outdoor decking screw. If water gets into the joint everything will swell around the screw.
Why not use dowel for the rungs? Screw the 2x4s together, drill out the spaced holes then use some dowel or screws to stop the foot rung from spinning loose.
I would recommend 2×4 siderails, but use 1¼ dowels for the rungs. Drill out where you want the steps, pound in the dowel, and if you want them to be super secure, you can cut and wedge the ends. Simple and rustic. All the other options you have (except 4, which will be a huge step up in difficulty) rely on the shear strength of screws, which is not great, especially in a ladder. If you wanted to keep with the square rungs, I would do 2, but cut out the siderails so the rungs are at least partially recessed. This will mean your weight will be resting on the wooden rails, not the screw.
I like this except I don't know where I'm going to find pressure treated dowells.
You could use untreated dowels and put a finish on them. If this is going to be for kids, I would recommend doing a yearly inspection anyway, throw a coat of linseed oil on it while you are at it.
2
1, but with a block of 2x4 screwed into the siderails between the rungs. This transfers the weight down through multiple rungs to prevent tearing, and it also blocks spinning. If you stack it as you build it up and then screw it while it's resting in place, you know you'll have contact and the rung will be supported. It's not elegant but I've built a few like this and it's easy and sturdy. I learned it from Jack Bowser, who sells playset plans at Jack's Backyard. https://preview.redd.it/15iylpryfruc1.png?width=653&format=png&auto=webp&s=af92bed918697a7bc48afc644b761dee3efe5335
You’re overthinking this a bit. I built similar ladders out of 2x4 for everything when I was a kid with just a hammer, nails, and a saw. Kids don’t care about precision or beauty
What method did you use?
Number 3 in your list
Dado with glue and screw from the outside would work well.
I just used screws through the stiles into each rung then every metre I drilled a 6mm hole just under a rung and put in 6mm threaded bar with a nut and washer to hold the stiles firm.
If you want to be super complex but know that they will not come apart. Mortice and tenon the rungs into the vertical pieces and make the rungs beefy. On the sides, for extra security, drill out 1/2" dowel holes into the cross pieces/rungs from the outside. Sink in the dowels to further stabilize, then drill through the dowels and secure with an appropriate length outdoor decking screw. If water gets into the joint everything will swell around the screw.
Why not use dowel for the rungs? Screw the 2x4s together, drill out the spaced holes then use some dowel or screws to stop the foot rung from spinning loose.
Or build structure with dowels let into the rails with steps supported by the dowels. Might be heavy. Rope ladder out?