One of the nice things about 5E is how easy it is to tweak. Double the XP to level, half the amount of XP from monsters, give XP for non-combat encounters (social, exploration, and quest), and this will give a feeling of taking a while to reach those higher levels, while not wanting to fight everything. Reduce the healing from a Long Rest to only Level+Con modifier, and require a Wis/Medicine check to get HP from HD during a Short Rest, which will keep a lethal level, as HP will be hard to come by without spending spells. Damage equal to your Con score while dying kills you instantly, rather than you full HP, and remove the ability of the Healers Kit. Remove Revivify, as that makes death less relevant at a lower level, and don't allow potions of healing to be purchased. Double the damage from traps and poison, and use cumulative falling damage (20ft = 3d6, 30ft = 6d6, etc.) to make exploration risky.
Using these suggestions, you return to a feel of old school AD&D, where death lay around every corner.