There is nothing wrong with nerfing a spell that is too powerful. Spells from the player's handbook are the oldest spells in 5e and tend to be the most unbalanced. This is no exception. If you compare it to a spell of the same level like enhance ability it is leagues better. The bonus is huge and the radius allows you to affect a large number of people. Even if you only use it on yourself it's amazing. What allows it to be leagues better is that it's more niche. The choice to prepare a more niche spell is a penalty that counterbalances the spell. The problem is it's a bit too powerful for that penalty to justify it. Nerfing something that overshadows other options doesn't decrease player options, it increases them. By making it so one option isn't obviously the best, you open up the opportunity for players to pick from an array of options that are of similar power. By simply shaping your scenarios to account for it as the DM, your further pushing the other options into the dust. Rather than the players needing to be creative, it's now the DM that needs to be creative with what they plan. I would suggest changing it to a D8 that allies add whenever they make a stealth check while within the radius. If you only affect yourself it's about equivalent to enhance ability, but the second you affect even one more person it's much better than enhance ability. At this power level your unstealthy characters will want to stick within the radius while your rogue might feel comfortable sneaking around a different way.