I think the $5 stiffening solution is a great way to start out, and I’d be very interested to know if you see any kind of a difference in cut quality before and after. If you can find or make braces for the Y axis that don’t obstruct any moving parts, that’s even better. You may even be able to cut out aluminum braces similar to mine (https://discuss.inventables.com/t/y-axis-braces-and-belt-dust-shield/15681) with the original spindle after you get the X axis stiffened, although I’m hesitant to recommend that since it would take a LONG time.
The 611 does require a new bracket, and the 611 itself is ~$110 USD, so it’s a bit of an investment. I encourage upgrading to it when you can afford it, since you’ll ask yourself why you waited so long the first time you use it. I did a simple before and after comparison with the original spindle and the 611 (https://discuss.inventables.com/t/24v-spindle-vs-611-router-quantified/15753), and the cut time dropped from 105 minutes to 6 minutes, although a more ideal speed with a cleanup cut would have taken it up to 14 minutes. If you want faster cuts, replacing the spindle will make your day.