To be clear, I meant a single character disengaging and falling back so others can deal with the threat.
But you can gauge from the hits you've taken if you want to risk 1, 2, 5, or whatever more hits. Players have to judge for themselves when that point is.
YMMV of course, but I have never found combat healing to be bad or unrewarding, a lot of players like that role IME.
5E is easy using the default adventuring day/encounters/etc. once you reach a certain point. Yes, you can throw infinite dragons blah blah blah but that isn't the default design.
I don't find out of combat healing to be super good, either. It is just another form of healing.
One of our golden house-rules might help you here: Upcasting spells yields maximum results on additional dice. So, in your example, instead of 3d8+3, make it d8+19.
Fight the monster? I don't know, there are too many variables to answer that. PCs get a lot of hit points as it is, and self-healing in combat just aggravates the issue as I see it.
There is also where players have to be responsible for their own defense. Disengaging and Dodging are two of the least used but most effective actions IME. You heal one, the other focuses on defense while waiting for help.
FWIW, our first 5E didn't have any sort of healer-caster. We had a PC with the healer feat, but that was it until after 5th level, when a druid finally joined the group.
5E offers a lot of out-of-combat healing. So, yes, the point is to make it through the fight to use some of those healing resources and move on to the next fight. There are lots of solutions if you want to it easier, like 5-minute short rests.
There are plenty of other fail states, dying is just one (and arguably the most effective) fail state.
So, what do you want the fail state to be?
It's a good question, that. I don't really have an answer, because I've been too focused for a long time on "what could go wrong" and trying to adjust for it. When I run a game, I spend a
lot of time planning sessions. I go over the monsters, I compare them to the party's abilities, I try to make encounters memorable, throw in a few non-combat challenges, weave an interesting story about the area they are adventuring in, and generally, spend more time planning than actually playing.
When things go wrong, it's not usually a little wrong. Like, oh, we might have to skip an encounter or whatnot. It's usually catastrophically wrong, and it always irks me when I say "this happened to me" and other people are like "lol, impossible".
No, it's not impossible, and it can happen. I've had a player up and leave my house when they died in the middle of a challenging encounter, realizing that they weren't going to be able to play their character for the rest of the night.
I've had good campaigns that I'd been running for over a year suddenly crash and burn, with players all of a sudden going from "we should play this weekend at the last minute, bonus session!" to "uh, I'm too busy at work, I can't play this week" all over a session that went horribly south.
There's a limit to what kind of abuse a lot of players can take, I've found, so it's all this careful balancing act to make sure that I know what they can handle, and just have to hope they know what they can handle. Personally, I think part of the problem in communication is that, in games where you're actively trying to challenge the players, the players are probably better at strategy and cooperation than my usual pack of misfits, and probably have more optimized characters as well.
My groups are...a mixed bag. I've yet to have a successful session zero, for example. For 25 years, I've tried bringing the players together to plan their characters as a group. Nope. Best I get is "whose the Fighter? Who is healing? Ok."
Then they all make their characters independently of one another, and you better believe they are all at different power levels from one another.
And the guy who said he'd play a Cleric comes in with a Storm Cleric who wants to blast enemies to bit with thunder damage rather than heal (imagine that!), and the Fighter is a janky dual-wielding Eldritch Knight with 12 Constitution...
And this is what I get to run for, lol.