When doing all that you are doing, chronic thrush is not the normal, "he got left out in the mud, or the stall wasn't cleaned" issue. It's an immune system issue. I could say how I know that, but I won't
You will probably have to change your game plan starting with the feed
All the starch and sugars that go into a horse's mouth, also go straight to their hooves; along with causing problems in the hindgut.
Regarding the Purina Senior: The best horse feed: A closer look at what to feed your horse
Where it says in part - even though it is talking about what makes a horse hot, take a look at the high starch value (or non-value:) of Purina Senior:
We’ve all heard that grain can make horses “hot”. It turns out that it is the non-structural carbohydrates (NSC) in grain that does this. Different grains have different amounts of NSC. The highest are corn (60%) and oats (52%). By comparison, here are the NSC content of Purina feed concentrates: Strategy 30%, Senior 20%, Wellsolv 11%, Nature’s Essentials Enriched 32 0%.
If your horse is getting the recommended amount on the bag (or more), the NSC value is off the charts for him.
Before folks come in and say "my horse has eaten senior for years without issue", it isn't "your" horse that has the problems. Every horse is different. I have four. The one that used to have the chronic thrush problems is oat/corn/soy intolerant<---intolerant. It shows in his attitude, it shows in his hooves.
I only found that out by accident after my senior TWH was diagnosed as insulin resistant four years ago and I started feeding everyone his bland diet. What a change for the better that made.
My horses come in at night and even though I clean stalls every day, I have found that grid mats in all the stalls has helped immensely. We re-did the stalls last year with several inches of limestone crush, grid mats (the ones with the holes) next, and shavings that get changed as-needed are kept on top.
Between diet change, good trimming, re-vamping the stalls, I have gone from having my nose in that horse's hooves every day to putting Absorbine Hooflex "Thrush Remedy" in his hooves ONCE every 2 - 3 weeks as a preventative. It's absolutely unbelievable.
This stuff is a hoof miracle worker. It keeps the bad bacteria at bay without compromising the healthy tissue. It does not dry the hoof out. Hooflex® Thrush Remedy – Hoof Care – Products – Absorbine
This horse also has dust/mold/pollen allergies (does the OP's horse have any indication of those things - just curious
The feed program that works to keep this horse's hooves healthy:
1. Rice bran
2. Omega-3 Horseshine
3. Equi-Pride vit/min supplement with a pre-probiotic in it (soy free
4. People Vitamin E gel caps
5. Allergy Herbal blend seasonally but that doesn't change his hoof quality
6. Quality, weed-free mixed grass hay.
7. Pasture turn-out daily, all year long
This horse also has a less-than-Grade 1 club hoof that is REALLY prone to thrush, but it has been thrush-free since I took him off oats/corn/soy.
Even though I can't find anything, I am sure Purina Senior has oats and corn in it - 20% NSC value is not safe for a healthy horse.
For all the research that goes into Purina Feeds, something happens between the time R&D finishes its projects and they go to production because Purina doesn't make anything I would feed my horses.
I tried the WellSolve L/S on my older IR horse when it first came on the market. He became "off" enough (lethargic, slow moving) that I knew the other half of the bag was going back to the feed store and I went back to feeding rice bran and an additional vit/min supplement.
IMO, 60% of the hoof success will be in your farrier. I hope when you say you like him, that doesn't mean how he interacts with you but the kind of job he is doing with your horse. He can make or break those hooves. I never have cared how the vet or the farrier talks to me - just make my horse better
40% is what goes into the horse and his living conditions.
To repeat myself, this horse doesn't have the typical thrush that most folks are used to dealing with; when it just won't go away no matter what the owner does, something else needs examined starting with diet, including supplements.
I hope this helps and I am sorry I wrote a book