So I think the problem with that model is "Tough" actually has two aspects to it in 5e:
- Ability to take lots of pain
- Ability to reduce needed healing.
The fighter for example, is tough because of its high AC AND the fact that it gets a free heal every short rest. The paladin gets lots of healing. The barbarian can soak lots of damage by halving it with rage. Even the rogue can do a similar soak once per round.
The monk.... has none of that unless you take the Open Hand style. Its AC is not particularly great, but it also has no healing recovery. This means that a healing in a group with a monk is going to get tapped more than if they had a fighter.... monks are healing sponges.
So increasing the Monk HD to d10 does give it a little more toughness, and a little bit more recovery on a short rest...but they are still damage sponges in comparison to other front liners.
The two abilities I've been trying in my own homebrew lately:
1) Patient Defense: (remove bonus action) Spend 1 ki as a reaction to an attack or dex saving throw.
2) Meditation: A monk uses an action and begins concentrating, like on a spell. After 1 minute of concentrating, they regain hitpoints equal to their martial arts die + proficiency bonus. This ability can be used a number of times per day equal to the monk's proficiency bonus.
The first gives the monk a solid AC when they need it (without spending KI just for the risk), and doesn't interfere with their offense. Meditation gives them more recovery so they aren't as reliant on secondary sources, but not in a way that clashes with the fighter or paladin, the healing is good but slow.