First, plums are excellent for bonsai. They adapt well to pots, can be pruned and back bud well and tolerate hard root pruning. Also real tough. On top of that you can get flowers and fruit. All wins for plum as bonsai.
Tough as nails, which tallies with the self sown seedling origin, they don't care much how you treat them. Water, fertilize, trim when necessary.
Taper is important in many bonsai styles but no one aspect is so important that other aspects should be discounted, meaning you can have a good bonsai that lacks taper provided everything else is great.
Just looking at the photos I think you may be giving the trunk bends a little more weight than they actually have so maybe need to reconsider taper V bends. Knowing that plums develop pretty quick I would not hesitate to chop at least a bit.
Depends what you expect from your bonsai but to me it is still just a stick. Trunk can't be much more than finger thick so it is probably only a couple of years old so far. I'd probably let it grow for a few years to thicken the trunk then cut back and develop branches but if you are happy with a thinner trunk you can move right to branch development so wiring the new shoots and trimming regularly to build up ramification.
With the bent trunk it probably suits informal upright style but could also adapt to windswept, leaning or 'natural' style depending where the branches occur.
