I’m afraid there is no better way to do it, as far as I’m aware (and I’ve done quite a lot of statistics over the course of my degrees). Without rewriting the whole attribute system from the ground up there’s no way to keep the increase in averages and deviations similar as you level up; this is why I suggested either finding a different system or making a new one.
There’s some good news though, you can go beyond level 10 in Open Legend already! The attribute cap doesn’t increase, but due to the costs of the higher levels it would still take another thirty seven levels before you run out of non-Extraordinary attributes to level up. If you count the Extraordinary attributes it goes to level eighty seven. I’d say it would probably be around level 25 or so when the characters start to look similar, which at an average of 1xp every 3 sessions (about what I’d aim for in a long campaign) and 1 session every week would keep you going for about 4 years. The PCs will be massively overpowered by this point of course, but much less than if the attributes extended past 9. If you’re not sick of Open Legend after that long playing it week in, week out then you’re a bigger fan than I am
I would expect some prices to be wrong to start with under a new economy. Don’t be afraid to keep tweaking it as you go. As for your other point… there absolutely is somebody stopping a player buying an infinite number of WL 1 items: the GM. You control how many of an item is available to buy, and you should also consider that buying in bulk is subject to its own economy if it ever comes up. There’s a reason a sword is WL 2, armor is WL 2, but outfitting a small militia is given as an example of WL 5 (not WL 3 as you seem to be assuming).
This is a point that comes up a lot with people new to the system, and I’m not quite sure why. In D&D the suggested price for a chicken is 2cp, but I’ve not once seen anyone complaining that a player with 3,000gp can buy a thousand chickens without scratching their wealth. The WL system is designed so that if something is cheap enough to be inconsequential, you don’t bother keeping track of it. Like most DMs won’t make their party pay for food and water in a D&D game, unless your party enjoys tracking silver and copper. The WL system just says “Is your supply of money an order of magnitude greater than the cost? Then yes, you can afford it no problem.”
Having gone back and reread it, I think I understand it now. If I’m interpreting it correctly, you want a much wider range of armor to be available and have written up some formulae for calculating how much they cost. What I wasn’t getting the first time was why that wasn’t in the section on items, having it in character creation confused me a little and made me think the change was based on the character rather than the armor they’re wearing.
I still think that allowing the WL reduction is dangerous, because it allows high Fortitude characters to wear ridiculously strong armor. I also think you mean “speed penalty” rather than speed when adding and subtracting from the armor, which causes huge problems in combination with your other rule of swapping HP for speed. Specifically, it allows high Fortitude characters to spend some of their plentiful HP to increase their defences which reduces their need for the HP to begin with as it reduces the damage they take on each and every hit.
Overall, I think there’s some neat ideas in there but a large portion of it just seems to be homebrewing for the sake of increased complexity. Maybe I’m just not seeing the end goal, but some of the things you’ve written up here (especially the action system) don’t seem to add anything to the game except busywork and more things to remember. Complexity is a cost you pay to add what you want, not a goal in itself.