I've found that thickening the trunk is the quickest part of developing bonsai. Much more time is spent healing chops and developing branching and ramification. You may have cut 2-3 years off starting from seed but I'd guess the roots and pole trunk will add 3-5 years onto the project over a trunk that was developed more carefully.
Layering is of the most oft suggested panacea for anything bonsai but IMHO it is more often a waste of time. If you can get cheap seedlings they will graft roots just as easily and you can start immediately instead of next year.
The blue layer line appears to target a fork in the trunk which may give a reasonable start to another trunk but I don't think the shape of that fork is as good as you hope.
Your proposed chop (red line) appears to be quite high up the current trunk. How tall do you plan to make the bonsai? To get good trunk taper, good bends in the trunk and good branch placement I find it better to make the initial chop around 1/3 of the anticipated height as we need quite a bit of growth above the chop to heal the wound and make the final apex.
I have grafted quite few roots on tridents. Sometimes it works really well but there's often a scar left after removing the top of the seedling after graft has taken so you should allow for that in your time lines too.
I have not tried thickening a trunk by grafting onto the outside but I have done some trails with fusing seedlings together. Not every one ends up looking great so I would not pin all my hopes on one single try.
Good luck with your trident.
O that's interesting, i knew chops took a long time...especially larger ones, but regarding the trunk thickening not taking the longest, is this specific to faster growing species like the trident maple or does this apply to slower growing species as well?
So the layering bit is more of exercise of learning, as i've read most of the literature but have never applied it and the way i see it i would rather try this out on a bunch of cheap $20 trees then trying it the first time on a $1000 tree (doubtful it would be needed, but people are crazy)
Yes, so i realize the red line is high, in my heart i would love this tree to be 1m tall, but realistically i live in a small apartment, so big trees like that won't work, realistically i'm thinking it would be about a 50cm tree possibly 30cm. So i choose that spot as it is the lowest point that still has a branch and leaves, as i'm not confident enough to make a complete trunk chop with no available shoots. I was hoping this would be a nice stop gap approach to cut it and then if additional shoots appear lower cut again
Ok, great to know it is a possibility at least, rather than just wild hubris. I'll have to see if i can find some sapling from a nursery or somewhere, but this is going to be tricky, as i've noticed that nurseries here tend to not sell their saplings often, as obviously they cannot charge as much as they do for a full grown tree