I am strongly considering that my next campaign use the Gritty Rest variant, or at least a close variation on it. And I'd like your thoughts on how this will impact play. I've got a couple of thoughts I'd like to address already, but I am even more looking forward to your thoughts, especially if you have run Gritty Rest before. My goal is to fit encounter-per-long-rest to my DMing style, with 5-8+ encounters between long rests. So this isn't "lots of encounters per long rest", this is "the correct number of encounters per long rest".
I have considered doing the same, but had not gone as fully into the details as you have considered. My plan was to have a Long Rest be either 3 full days of rest (no casting, traveling, combat, etc.) or part of a week of downtime. I figured the players would choose it as part of downtime more often, and it would make downtime be an important part of the campaign. The 3 day rest option would be while on quests and any time based adventures, where they might have a chance to rest, but not enough for a full week.
Short rests per long rest
First the ratio of short rests (overnight) to long rests (7 days) will be rather off, with a lot more short rests. This will empower short-rest-recovery classes like the warlock, the monk, and fighter (battlemaster). Actually, that's not true. I think those classes will stay in balance with the at-will classes, but both will have an advantage over the long rest recovery classes like the casters, as well as the hybrids like the paladin or the barbarian.
Are there any (sub)class or racial features that I should reduce uses of and move to per short rest? For example, halve number of barbarian rages, but restore one used one on a short rest? I'd mildly prefer not to do this, but will if needed.
I don't think any modification would be necessary. While you're more likely to see short rest classes, the number of short rests per encounter may not necessarily change. One day might have 2-3 encounters, while another might not have any at all. The big advantage for short rest classes is that they
will have to actually happen.
Long duration spells (& rituals)
Some spells are meant to last for multiple encounters, or for "all day". With fewer slots, do I still support these intentions? And where's the cut-off for some things are still back to back. For instance, 10 minutes I wouldn't touch, but 1 hour I could make 4 hours. Do I do this for all casters, or all Spellcasters and leave Pact Magic alone?
For the all day ones, do I make them them into rituals? If so that gets rid of any slots used on them, and expands usage like using Mage Armor on every light armor wearer. Plus it doesn't handle spells like Hex having a longer duration when upcast. Perhaps I should just increase those as well. 8 hours becomes 3 days, 1 day become until next long rest.
Except for a handful of spells, spell duration is largely meant to last for 1 encounter. Even the hour long spells are seldom likely to impact more than 1 encounter unless you're in a tight location with lots of enemies close together (i.e. a dungeon). If you really feel the need to edit those handful of spells, you should do so on a case by case basis.
Personally, I feel that Mage Armor is a garbage spell, so it being weakened is a feature, not a bug. If you want to save it, I'd have it provide a ritual you can cast 1/day for the next 7 days or until you finish a long rest.
I wouldn't worry about Hex, since Warlock is going to get spells back every day anyway.
Magic item recharge
If you consider a week of adventuring and then a week of downtime, that's 14 days. I was thinking on making magic items that recharge do so on the new and full moons.
This is one I haven't considered. I would suggest making attuned items recharge when the owner finishes a Long Rest. Other items might recharge every 7, 14, or 28 days, based on how often you want them to recharge.
Encounter strength
While a Medium encounter doesn't feel like all that much, it it attrition of resources. This is just to remember that Gritty Rest will likely ensure the game will be heavily attrition based and that even lesser encounters will have repecussions, and that limited resources spent avoiding an encounter might be best.
Ignore the DMG for this one, and use the encounter matchups in Tasha's. It gives much better fights that are balanced against the PCs. One thing to consider with the number of encounters per Long Rest is HD, so you should error on fewer encounters per Long Rest initially.
What have I missed?
I don't know what I don't know. Outside of frequency of usage which I think I addressed, what else should I be looking at?
So, your thoughts? Especially on what I've missed.
Something to be aware of: casters are going to rely on cantrips a
lot more than normal. Spell slots are going to be a premium resource, so casters should be much more stingy with them. Since players aren't used to this, you're going to want to bring this up in session 0.
Because of this, remind the players of the adventuring gear in the PHB. Not just Potions of Healing, which are going to be a necessity too, but Alchemists Fire, Caltrops, Crowbars, etc. Things are likely going to be overcome more with checks and smart play than with spells. Casting Knock is a waste if you have a portable battering ram.