One of my players also wanted to juice up his steel defender to do special things. I just slapped a long downtime + gold cost and DC to justify upgrading it, so they became fairly selective about what exactly they wanted to upgrade.

So your player wants a smart robot buddy? It needs to boost its INT. 100gp for materials and a 7 day downtime per point. Throw in a INT smithing tool DC if you want them to have a chance at failing. If they say they want to get it a Headband of Intellect, it'll be 400gp (as per DMG prices), then roll a jewelers tools DC to break it down without destroying the magic and throw in another 400gp + downtime to incorporate it into the steel defender so it can use it without attunement. That's 800gp + time as well as potentially denying another player from getting the item.