I haven’t read any of your previous posts; just going off of what you’ve posted here.
I’d start with a neck x-ray. The reason is your comment about his legs falling out from him or tripping or otherwise acting funny on the lunge.
Last year, we had a horse diagnosed with cervical vertebral compressive myelopathy at 8, after many wins in flat classes, over fences classes, etc. He did some weird things in the round pen - but you almost wouldn’t notice unless you watched. He would turn his neck a specific way, and it was like a nerve was struck in his legs, and he didn’t know where they were for a split second.
The neck X-rays would be cheaper than the bone scan and may give you answers that straight retirement would not. However, I have a horse in my pasture who is a walking nightmare of body issues - kissing spine, bad hocks, bad feet, bad everything… and he’s been retired since he was 9. I adore him but bless his heart, he was not born with a winning hand to stay sound and comfortable with a rider - so we keep him out as much as possible and when he tells us that he’s too uncomfortable even for that, we’ll say goodbye.
Best of luck!