I think you feeling strange is kinda the problem here, rather than anything else.
You have four characters who can output a lot of healing. Druids can heal crazy amounts (esp. if you haven't nerfed Healing Spirit and it sounds like you might not have - otherwise it heals for a joke amount), Clerics are really solid-to-great depending on subclass, and Paladin is a whole chunk of healing. I don't see your total party size or if anyone else can either heal or generate THP, but I presume there's a bit of that too.
Given that the biggest death-risk in 5E, is starting a fight on low HP, getting downed instantly (then either taking enough damage to die instantly, or taking multiple hits which force extra death saves), and they have four strong healers in the party, why on earth would they be starting fights, intentionally, on less than maximum HP? Sounds like you're mid-to-high level, too, so they have a huge depth of spell slots to pull on, too.
Add in that they're using HD and potions sensibly, and it sounds like what you're complaining about is a big fat case of "Working As Intended". A big clue is in the healing potions. If they weren't being pressed at all, if they felt totally safe, they wouldn't be using consumables like that. They'd just stay in their bags to rot.
I don't think you need to change anything, just get on board with the fact that the party is healing heavy.
We did nerf Healing Spirit as it can only heal 1d6 once per round (so 10d6 over a minute) and we use it out of combat mostly. 35 HP for a 2nd level spell is still a lot, but the time requirement balances it out somewhat IMO.
Yes, we are levels 14 or 15.
The healing potions are used nearly only out of combat, to save spell slots, lay on hands, etc. just to top them off. Of course, using Beacon of Hope and 10 healing potions restores 100 HP in a minute.
You're not thinking like a player.
A player doesn't want their PC to die. You're sneering at 30-point hits, but they could be crits for 50. And four thirty-point hits landing on a 110 HP character means they're down, and in need of massive healing.
Also, did they start around 10-14, or did they level to there normally? Because at lower levels, and awful lot of hits are 15 or 20 HP when you only have 30 or 50 HP, and it's only as you get to higher levels to the amounts of damage tend to seem relatively lower vs HP totals. So they'll be playing cautiously because they remember how it was.
You say "Oh for a dangerous encounter you want to be on full HP! But medium and hard aren't a challenge!". Well, buddy you got two separate issues here. And both of them stem from not thinking like a player. A player doesn't know what the "difficulty" on the next encounter is going to be. They have to assume every next encounter is Deadly. Not sure why you're not getting that. So they heal up to full, because it would be outright stupid not to, if they have the opportunity and it doesn't blow too many resources (and they seem to managing resources extremely well). And guess what? Medium encounters at 12th level aren't challenging. At all. There's no chance of death or real risk. That's how they are. Hard ones are rarely much more threatening. But they do drain resources, and that does add up. Players feel the challenge, as people have been trying to explain, because they see all the boxes for spell slots filling up, and their HD going down (and remember, you only get 50% HD back on a long rest). But DMs can be like you and be like "Pfffft you guys are all on full, whatevs...". So accept that medium/hard are there to burn resources, more than to provide "challenges" or the like. They burn a lot more resources if approached in a bad way.
Oh, those fire giant hits hurt, but when your front-liners average 150 hp, with pretty good ACs, it isn't that bad really. Especially when you know after the battle is over, you'll get topped off again.
We started at level 1. Even then it wasn't too bad, but again many encounters the PCs began with max HP.
I am
both player and DM. I get it. When I play, I don't know when a deadly encounter is coming, which is why I've said in other posts we top off whenever we can simply because of
that. As a player, I don't really feel the challenge in (most) medium and hard encounters. Sure, I might use a spell slot or two, or heal people up afterwards, but not so much I stress over it.
When I DM, I see the other side. It seems like a waste of time to have encounters
simply designed to drain resources. Now, don't get me wrong, from a story and "realist" point of view, it makes perfect sense. Not everything running around out in the world will be a threat after all.