This is broken. Completely.
If an ally within 60 feet of you would fall unconscious due to damage, you may use your reaction to cast Healing Word at 1st level without expending a spell slot. This spell must target that ally. You may do this a number of times equal to your wisdom modifier (Minimum of Once).
If people are going down more than 5 times per long rest something is going very wrong, otherwise this feat completely prevents player death. That is beyond broken.
Of additional note, the ability of Healing Word to get people back up easily is something that a lot of people hate, and I have heard the spell being banned entirely. At least in the usual situation there is a chance a player will miss a round, or get killed before the spell goes off, this removes that possibility entirely.
When you cast a healing spell with variable results, first roll a d10. On a roll of 10, your spell heals for the maximum amount.
Meh, a 1/10 chance of a few extra points of healing isn’t much
You may add your Proficiency Bonus to your Spellcasting Modifier for healing spells you cast.
Also meh, at level 20 getting an additional 6hp is barely going to register
Some parts of this feat are so weak I wouldn’t even look twice at it, other parts of this feat are beyond broken. Basically this feat gives your party immunity to death, but does very little to make it feel like you are a good healer.
I wouldn’t let this feat anywhere close to my table.
Additional notes on healing in D&D
Healing in D&D is pretty poorly done in my opinion, a first level spell slot in the hands of a cleric can cause 4d6 damage (Guiding bolt: average 12) or heal 1d8+wis (Cure wounds: average at level 1 probably 8).
So immediately causing damage is 50% more efficient than healing magic, never mind that damage can be caused via mundane methods and healing can’t (and I don’t count the healer feat as mundane, because a feat is expensive).
Add to this, a character with 1hp is as useful in combat as a character with 100, so the best use of healing magic isn’t to repair damage, but to rescue the action economy by using healing word to save a downed character.
The downed character then takes their turn as is nothing happened.
You seem to have identified this, and focussed your feat in that area, but what you are actually doing is exacerbating the problem, and just hiding it by removing the few seconds in unconsciousness. This actually makes wasting a spell slot (and the powergamer in my always thinks healing a conscious character is a waste) on someone who is still conscious even worse, despite your intentions of buffing the amount of HP gained.
The only saving grace at the moment is that during the moment of unconsciousness the character might miss a turn, but even that depends on the initiative order, and a well timed healing word already relieves the problem. Again this feat removes that possibility.
I would focus on what healing is supposed to be, which is something that keeps people properly healthy, rather than just keeping them conscious. How I would do this, I have no idea, but then again I would never waste a spell slot on a conscious character in the first place.