Well die, honestly.
For LoSH to really do much of anything, Clark can't be around anymore. I like that he's a legend from the age of heroes, Earth's greatest champion, but he also died like anyone else. Making him immortal effectively ruins his impact inspiring others to take over because I just don't think Clark would ever stop so long as he's able; why wouldn't he still be Superman in the Legion time? He has to go.
And I don't know, I just don't like Clark outliving Lois for so long and treating her as his greatest love if she's only a part of his life for what's effectively like 8% of it. A lot of Superman's themes and elements work best for me when he lives a shorter lifespan and him living longer is just more because it's cool on its face. Like yeah, it's dope that he lives forever but what does it actually benefit? It just prevents the DCU from getting to be more unique because Superman must always be there and it always has to accommodate him. Let his death be the end of the age of heroes and the beginning of the new age.
But Legion? Jon? Kamandi? Every future story? A lot of things require he not be around.
If he's around, I guess he just is an older Superman doing the same stuff he does now, only without any of his supporting cast-- it's just a watered down Superman for me. It can work for some stories, and it's been done very well, but for my money, Clark lives a relatively similar lifespan to others. I know this is a very unpopular opinion, but I think it also makes the time he spends helping others more meaningful; it's easy to be patient with others when your time is limitless.