Letting the trunk move with the breezes will do the most to strengthen it. It was grown in a crowded nursery plot, and tied up to get the maximum height - which came at the expense of width. It WILL make up for this, but be careful in your tie-ing. If it is too loose, it may break at the tie in a very strong wind, and if it is too tight, it won't move enough. As well, as it thickens, you need to loosen the ties so it can continue to move. Using 2 or 3 ties would be better than a single tie - the wind-forces are better balanced. If you search, you should get several discussions of tree ties/supports and how to arrange them. Fertilizing, as long as the tree is planted in reasonably decent soil, will do little to help. Watering the tree as it needs moisture will be the most help, after careful monitoring of the ties.

Out here there are also ways to anchor down mid span areas of the joisting as well in crawl space areas that involves pumping in concrete and embedding anchors with strapping that bolts through the joisting. Here in the mojave desert of ca., it's fairly common practice on old homes before the onslaught of concrete slab foundations, which are pretty much the only foundation type you see here these days. Even with slabs though, back when I was framing many moons ago, wall to slab connects were "j" -bolts in the slab that the mudsill bolted down to. Unacceptable practice these days. You can still use "j's" but also have to have simpson strapping from footing up the wall 24" in strategic places depending on load, up lift requiremnets, seismic, etc. Spot on checking with references and it might be a good idea to check and see if that work was engineered and approved via inspections. If not, you might consider a consult with a structural engineer for a few hundred bucks. Might be dinero well spent!

Weak and thin seedlings indicate that they do not have enough light. Seedlings will naturally try to grow taller and taller if they don't have sufficient light hoping that at some stage they will reach the light before they die. The best idea is not to develop the weak seedlings in the first place rather than trying to strengthen them but if you give them lots more light they may improve. I'm in the southern states and so can't advise you on how much sun is appropriate but I think I would try full sun to 1pm and then filtered afternoon light.

Can you post pictures? Preliminarily though, it sounds like it wasn’t getting enough light so is etiolated. If insufficient light is provided plants will stretch to try to reach whatever light is available. They grow but the result is lanky & weak. Provide better light and new growth should be stronger, healthier and more compact. I don’t think that the current growth will ever get any stronger, though. You could always cut it & try to root the cutting. The remaining stalk should send out new side shoots, but if not given more light, the new growth will be stretched, too.