The easiest way to emphasize your casters in the early game, is to throw in lots of relevant skill checks, particularly knowledge or spellcraft checks. Almost any spellcaster is going to have relevant skill modifiers, and it proves their place in the party.
A related option is challenges tailored to make spells useful. A small item you need is on the far side of a chasm? Summon Nature's Ally or Mage Hand can help with that. Having ambient magic, or an arcane trail to be tracked with Detect Magic also calls out your casters.
Also, it's been my experience that Druid is one of the better low level caster classes, with cleric because of free swapping in of Summon Nature's Ally spells, reasonable combat ability (3/4 BAB, d8 HD, not terrible armor, buff spells), and their selections of durable spells, such as Produce Flame and Call Lightning.
For the Archivist play to their skills and their Dark Knowledge ability. Throw in a fair number of the types of enemies Dark Knowledge helps against, and they should feel pretty useful.
There is no easy answer
This is a tough situation and the solutions are all generally things that can make players feel that their strategies are being specifically targeted. That doesn't leave you with a lot of options for how to address, but here are some considerations in evaluating what to do.
Talk it out
As has been discussed, talk about what's happening with the players. Managing so many creatures isn't fun for you, and that's totally reasonable. Let them know it's a cool tactic and can be used, but please don't use it all the time. Chat about what they like about it, what concerns you, and at least see if they can understand where your coming from and see if they'll choose to alter their strategies.
I had played a bard for awhile that used animate objects and it honestly got tiring for me, too. And it seemed too much, so I only used it when it really made sense to use it.
Summoning is tricky, the DM technically picks
Going by pure RAW, the character's aren't picking the creatures, the DM does. But honestly, that's not a lot of fun. I don't think I've played at a table where the DM has picked the summoned creatures. Using this in your discussion may be a reasonable tactic to show that if you wanted to press the rules-first approach, then you could allow them to summon, but that you choose (maybe randomly) the creatures. That limits the capability within the rules, but it definitely isn't quite the same fun/feeling for the players.
Nerfing the spells
I'm really not a fan of this, especially if the strategy and use is by the book (which it kinda isn't with the above, but you get the point.) Taking away toys because you don't like it can present it's own table issues. There are better ways to handle this.
Encounter design
This ultimately is most likely your biggest lever here. While you don't want to create every encounter that counters this strategy, it isn't crazy to start filtering them and also having the minions of the BBEG know the strategy to counter it.
Counters
The most obvious here is going to be bringing in monsters that are resistant or immune to mundane damage. Summoned creatures aren't usually dealing damage that bypasses magical damage resistance or immunity, so bringing monsters in with those traits nullifies the summoned creatures strategy.
Next up is area affect attacks. These summoned creatures generally also don't have a lot of HP (especially the tiny animate objects). Drop an AOE on the, and you'll wipe them out.
Environment design can also play a part here. Make it so it's difficult to maneuver or have room for the summoned creatures and the option to summon them gets taken off the table.
Keep everything as-is, but introduce Handling Mobs
Chapter 8 of the DMG (Thanks goodguy5!) offers some optional rules around Handling Mobs:
Instead of rolling an attack roll, determine the minimum d20 roll a creature needs in order to hit a target by subtracting its attack bonus from the target’s AC. You’ll need to refer to the result throughout the battle, so it’s best to write it down.
Look up the minimum d20 roll needed on the Mob Attacks table. The table shows you how many creatures that need that die roll or higher must attack a target in order for one of them to hit. If that many creatures attack the target, their combined efforts result in one of them hitting the target....
If the attacking creatures deal different amounts of damage, assume that the creature that deals the most damage is the one that hits. If the creature that hits has multiple attacks with the same attack bonus, assume that it hits once with each of those attacks. If a creature’s attacks have different attack bonuses, resolve each attack separately.
This attack resolution system ignores critical hits in favor of reducing the number of die rolls. As the number of combatants dwindles, switch back to using individual die rolls to avoid situations where one side can’t possibly hit the other.
I haven't personally used this before and, as always, talk to your players about it. This isn't about you as the DM using this for your monsters, but minimizing the player interaction with their summons. This may not be what they're looking for.
But having fun is the key
Balancing letting your players use the tactic they enjoy with challenging them and yourself is the name of the game. Let them shine, but also put them in situations where their go-to strategy isn't an option. Coming up with new strategies and working out how to handle an encounter differently can be fun, too.
Best Answer
Matt Colville has a good video on this topic: Range and Altitude in Three Dimensions, Running the Game #55
It notably advises:
You'll also need to consider the underwater combat rules, Dungeon Master's Guide p.116-119. Notably:
And in the Player's Handbook, p.198:
I would imagine underwater civilizations would develop more effective magic and tools, such as waterproof spellbooks (or an alternative to spellbooks), and variants of major spells (such as a cold or acid version of fireball, non-flat versions of area spells, and spells which are equally effective when you're not at ground level).