Hey Dave. I have a GEEETech i3 which is extremely close to yours. I actually have a fistful of different printers and have reworked each one many times just because I get my jollies off on it. From all of my reworking and experience here let me tell you for our i3's the best we can do is to make this reinforcement for the front section of the frame. The back half is already pretty robust. But this is what I have done with my i3 to make it strong: https://3dprintboard.com/showthread....3-32bit-Rework .
This bracing is every bit as good as anything else you will do that will re use those 8mm smooth rods. The 10mm rods will have a little less deflection but really what makes these machine precise in their movements is the expensive CNC parts. Like the linear rails. I have converted one of my machines to good quality linear rails and guides for just and I do mean just under a grand for the rods and rails. Bang for buck it is not worth it. These printed parts make the frame rigid enough to pick it up and carry it around one handed by the top of the frame only and still have the bed be level. This way is cheap and brings real results. For strength in your stepper motors you could look to a 32 bit control board like the SKR 1.4 Turbo and a set of TMC 2208 or TMC2209 drivers and maybe invest in a 24v power supply.