when dad worked on Arran estates forestry they used to use iron clamps that could be tightened. (Maybe in conjunction with a couple of straps above to pull it in higher up. (as an aside – somebody there was doing tree weaving I saw a few where they’d bundled the branches at a young age and they had grown into straight trunks with a twisted ball of branches on top – different – no idea why they did it.)
Bloke down the road with a leaning tree (on a sloping bank) had (sensibly imho) stuck a couple of acrprops on it in the current wet and windy conditions – so depending on surface etc maybe you could use that as well
Could you get a wide strap/wire high up tree to a ground anchor behind the building
?? coppicing – scalp a bit off the leaning leg, scalp the bark off the inside leg (so faces would mate) – pull together and hold – long term