Um, no?
Temp hp from different sources don't stack so your hp total only benefits from whichever one of Inspiring Leader, Heroism, Aid, or the potion of Heroism gives you the most, and that's not going to help a lot.
First, Aid raises your max hit points, its not temp hit points. So it stacks with temp hp. Plus healing "renews" it, so its not just hp now, it can be +5hp/fight throughout an adventuring day. And one Aid benefits 3x targets, so its way more efficient than it looks, especially on summons that instantly vanish at 0hp.
Second, Warding Bond is a a damage sharing between caster and target. The caster and target split the damage the target receives. So again, overlaps with temp hit points, and in some ways can amplify them as one attack applies against both the target's and caster's temp hp.
Leadership's temp hp lasts all day (or until you take a rest) and costs no resources other than time. Which means it provides protection from ambushes. At 4th level its 7ish hp, which on a 19hp horse can be enough to survive like 40% of fireballs even if they fail the save. At 10th it's ~14hp, which is almost double. Oh wait, cast Aid and the horse has ~38hp, which is double!
Then, when those are gone, you layer on other more transient and resource intensive defenses, like heroism and potions.
Heroism does require concentration but as it provides bonus hp to both paladin and mount each round (yay spell sharing!) it has a higher net benefit than at first glance. Paladins at 6th level should have pretty solid con saves when the aura kicks inso the concentration check is plausible. And again, it keeps an ally from going "poof" at 0hp.
is this really what you're choosing to use your only concentration slot on, especially your bard or your cleric9/paladin1 who could otherwise be throwing 5th level spells around?
To your point earlier about temp hp stacking, concentration spells aren't needed until after the Leadership hit points are gone. Not every fight should open with a fireball. Many times those ~14hp (~19 with aid) would last through the initial "tussle with guards" fights, even in tier2.
The cleric9/Pal1 wasn't a mounted combatant. They were a demon-banisher who has a griffon mount via an expensive scroll. Leadership+Aid turned a 59hp steed into a 79hp steed. That survives most ambushes, and the slayer would dismount and let the steed retreat to protect against flankers or rescue people by pulling them out of combat. Add a zero-concentration 1st level Sanctuary and the steed was pretty durable, surviving multiple encounters/adventures per scroll.
The Paladin would vary between mounted and not. In the window between scrolls and having Greater Steed at 13th, the greater mount was more a way to get to enemies fast and then dismount, because scrolls are expensive. At 13th it was a lot more Leeroy Jenkins, as an 82hp steed is extra durable bolstered by mounted combat so melee attacks don't even hit the mount. Plus it could be resummoned later without a pricey scroll.
The Bard would mostly rely on range and mobility; drop a concentration spell like Hypnotic Pattern and retreat at velocity to cover. In combat the mount would get the first Inspiration to help it survive spells. At 13th the bard would share Eyebite, which turns the target's eyes black and lets them use their action to inflict a curse-like effect. Doing that twice a round made the spell extra useful, though once a target saved they were immune to both. Draconic Transformation was super fun when there were two breath weapons per round coming from the sky.
And as for "building around steeds", yeah, a bit. They are an expensive resource but also a useful resource. It makes sense to maintain them. By the same token, the same things that keep the steed alive also keep the squishier PCs alive. Leadership and Aid are virtually a +4 stat boost to Con, giving level+cha+5 hp and both affect multiple targets per use. That's a non-trivial boost to either give the squishies a higher floor or raise the ceiling on the meatwalls as those temp+bonus hps are like healing spells cast in advance.
The tactics become ingrained and expand. Is the rogue the only one with a chance of getting a mcguffin? Well, to be safe, let's cast Sanctuary and Warding Bond on top of Invis since we have them prepared. If things go sideways, the rogue has a real chance of surviving to flank the guardian for sneak attacks while we close (rapidly). Alternately, we do something similar for the paladin and have him grapple the unkillable (without the mcguffin) monster, letting the rest of us dash by unmolested, then we throw some short-lived nerfing spells to let the paladin mount up and flee.
Mounted combat IRL requires a lot of dedicated skill and equipment. Aside from saddles and weapons (that should match, a lancers saddle will drive a mongol insane and vice versa) you need war horses that are trained not to freak out when someone screams in their face. I owned horses for about 2 decades and leaves blowing in the wind can panic them. Realistically, non-war horses are a huge detriment in a fight as they should panic, buck, and rear at sword-on-shield, shouting and spells going off.
But the game mostly ignores all the downsides of mundane horses and makes you do some work to get the benefits.
