OK. Possible consequences and considerations for the 5 Round Working Day:
DM side:
Expect a major nova at the start of the encounter. If you let the party get to the BBEG then they will dump all their resources into them and probably kill them. Likewise control or damage spells against available targets. With no need to budget resources, there is little reason not to open with your most powerful effect.
Best to not let them access to the final target at first. Stage fights so that the party can't see the BBEG on the first round or cannot engage them immediately. Do not have a single BBEG, have several.
Spread out and varied enemies, so a single spell can't affect a large chunk. Mix in ranged attackers as well as maybe some undead or constructs that may not be affected by the same effects as the rest of the opponents.
Party will theoretically be able to handle fights that are much greater than the deadly encounter budget, but also much more random. A few bad rolls could end the campaign against that level of opposition, but if they don't roll badly, the party can probably roflstomp most encounters. Use a wave system for the encounter will let you keep a stream of opponents to challenge the party, but that you can cut off if it looks like the characters are in trouble.
(All of these will be heavily dependent upon party composition. An all-caster party will be in their element and able to handle much more dangerous encounters than fighters and rogues.)
Player side:
Massively favours specific classes and actively discriminates against others. Some will get their full expected daily resources. Others will only get a third of theirs.
Will favour
against concentration spells, since a caster con only use one, rather than 5-8 concentration spells they could in a normal day.
Player healing is heavily disincentivised since it won't be able to keep up with incoming damage. If the fighter drops in round 2 then it would be more effective to let them die and ress them after the fight is over than waste a round using healing instead of a top-end spell.
Concomitant much greater risk of death spirals and TPKs.
No HD-based healing possible. This would normally occur in the two short rests of the standard adventuring day and mean that the party can recover HP without spells. Without these, the party are operating off roughly half of the HP they would have over the course of the day.
What about beyond the immediate tactical concerns? How does this idea affect the campaign broadly?
I think there are a number of models that might lend themselves toward the nova game. PCs that are explicitly a special strike force, for example, or are undergoing long term travel that only allows for isolated combat encounters. What else?
More spells available for non-combat use. Teleports, Pass without Trace spell rather than stealth skill, Levitate or Fly rather than climbing, divination spells rather than investigating etc.
It occurred to me that another way to do this if you still wanted that multiple fight experience from a storytelling perspective would be to just say "all PCs gain the benefits of a long rest at the end of every encounter." That way you could still have a dungeon romp -- it just wouldn't be about resource management.
That would have a similar effect, yes. With all the associated risks and impact on party composition and player enjoyment.