If I were you, I’d go with a stallion that is proven to consistently stamp a specific phenotype, regardless of the mare he covers: meaning he makes carbon copies.
Because your mare is a first generation cross (F1), you don’t know what she might produce in terms of the foal – will she predominately pass on the draft? Sickle hock and dropped croup? Or will she pass on the TB, maybe downhill or with a poor neck? While she might look a specific way (phenotype), I’ve found that the phenotype is rarely a good measure in predicting the genotype that an F1 will produce.
I’d look for something that has a high percentage of TB or full TB, personally. TBxClydes are wonderful, but they’re F1 and it can be very unreliable breeding an F1 horse. Not because they aren’t good horses but because they tend not to pass on the same consistent traits - sometimes you get an F2 that looks just like the mother, other times it seems like random horses were pulled into the mix. The point of breeding is to consolidate specific trait[s] – when you have a first generation ‘hybrid’ of breeds, it can be very unpredictable on whether or not that F1 is going to produce or replicate what you see on a phenotype/genotype level.
There is a reason that many clyde/tb or shire/tb or even belgian/tb breeders (for foxhunting, etc) rarely take their F1 and cross it back to another f1 – typically they will cross it back with a TB or an arabian as both of those breeds are very consistent in what they pass along and there will be no surprises in terms of sudden phenotypes that may pop up that weren’t visible in the parents.
The best example I’ve heard or seen for F1/F2 argument: when you breed a pug to a beagle, the resulting puppies are F1 ‘puggles’ and are very consistently the same regardless of sire and dam: tawny agouti fur like pugs, moderate brachycephalism, long floppy soft ears, darker mask around the eyes, proportionate limbs. When you take an F1 puggle and breed it with another F1, the results are all across the board and very unreliable: sometimes you get spots, sometimes you get stunted limbs, you get extreme brachycephalism, etc. You almost never get a second generation ‘puggle’. Instead you get a mixed breed looking dog that varies from tawny to white/black spotted, some with good proportions and others looking like franken-dogs. The practice of breeding F1 to F1 rarely seems to produce the same phenotype as the F1s even though the parents may look the same, the genetics are still a very mixed bag.
Which brings me to the horses you’ve mentioned; I will let more experienced breeders chime in, but personally I would not be using a warmblood unless I had seen other offspring by draft/tb mares by that warmblood that were exceptional.
I might consider Gatsby, if only because he does tend to lighten heavier horses, and I have seen him cross well with draft/tb crosses. He will probably not improve movement, but he will contribute brains and good aptitude for LL eventing.
Other than Gatsby, I’d really strongly consider a TB. Most draftxtbs have a decent gallop, but when you cross them with a warmblood the gallop can be lost if it is not a warmblood specifically for eventing.
Of the TBs I’d consider:
Sea Accounts
Coconut Grove
Sea Lion
Salute The Truth
Chiron
Lucarelli
My personal favorite is A Fine Romance; unfortunately he passed away last month, but if he has any frozen he would be my top pick. Salute The Truth would be a close second BUT his progeny are varied in their personality - some are quirky and some are very tractable.
For warmbloods, I might consider Lotus T.
