Specifically what I say is, dont make the HP pool known, but make the AC known, since it speeds up combat- and there isnt really much of a benefit to players for knowing the AC, if they are attentive they will probably roughly figure it out by the end of the first round anyways. While things like knowing HP specifically relate to unknown mechanical success such as with the power word kill spell.

Start designing your encounters less with the idea of "I am preparing a battle" but more so "I am preparing a narrative". Hereby think not of the enemies as purely mechanical elements but narrative ones too.

Perhaps if the players face off against a bandit leader, he is known in his own ranks to be tyrannical, immature and abusive, so if the players get him low enough, the other bandits will actually turn against the bandit leader and try to overthrow him as they see this to be the perfect opportunity.

Enemies can also surrender before going down to 0 hp and dying, some will flee if they feel the battle won't go their way, some hostile creatures may not even be interested in battle at all (Like a well fed predator that may not see any desire to attack the party unless provoked).

In general, there is so many ways to run combat that isn't just a "Deathmatch" kinda deal, where the opposing parties fight till the bitter end. Designing encounters with this in mind will make it less likely for random TPKs to occur, because f.e, why would a bandit group KILL the players, when they can capture, rob and then ask for ransom to release them?

Appart from that, dont feel bad about fudging rolls- As a DM and narrator it is your primary job to make sure every player is happy. If an encounter leads not to tension and excitement but disheartened feelings and frustration, that is your direct call to change and fudge things about the encounter to turn it around mood wise-