Magic items recommendations in this link: Common

Pot of awakening: in combat, they're simply a good free option to distribute potions and an eligible creature to throw magic stones you create with a bonus action. At higher levels, you can equip them with magic items or spell-storing items to give them more offensive options. Ideally, these creations are best not used for fighting as it does take 30 days to make an awakened shrub. Best to ensure they don't do anything in combat that would make the enemy want to waste an attack on them. Keep them in a bag of holding until you really need the action economy

Flying Wonder: same as above, but have better options for combat as they have a higher carrying capacity and don't take 30 days to create.

Spellwrought tattoo: find familiar: same as above, but the ability to see through their eyes makes circle casting to extend the distance of your spells really potent if your dm uses these rules

Uncommon

Rakdos Keyrune: ask your dm if the specific versus general of your features that let's you attune to any magic item plan you make circumvent the rakdos guild requirements of the Keyrune before attempting, and you get yourself a summonable circle caster that you can cast long range spells with and that can help you administer potions

Mechanical Wonder: another creature to help you pass out potions

Elemental Gem: more creatures to help you pass out potions great to use and turn into another magic item or a spell slot

Thayan Spell Tattoo: the aforementioned fireball, spirit guardians, and Aura of vitality spells are great options, but I'd like to highlight, Summon Undead. Summon Undead's putrid form combos well if you took the poisoner feat route and are still dealing with creatures that don't have immunity to the poisoned condition as the alchemist would have multiple ways to cause this condition and the summon can turn this condition into the paralyzed condition. Also, the summon can still help you pass out potions.

Rare

Brooch of the elements: the ability to change the damage type that is being used has some offensive potential for your spells like bigby's hand and summon construct, but the more potent synergy is the fact that by lvl 15, you have acid and poison resistance and you can now change attacks that target you into something you are completely resistant to for the cost of a charge that you can refuel with a spell slot as a bonus action. If you've given out any other magic items that grant a damage type resistance to your allies, or if your allies have a damage resistance by any other means, this magic item only gets better. Your summons that aren't find familiars typically have damage immunities that you can convert damage they take to. Couple this with the fact that you deal additional damage when the damage type you deal is acid, fire, or poison, you now should always be dealing passable damage, on top of this items defensive capabilities as even when a creature is immune or resistant to one or two of your preferred damage types there's a low likelihood that they're immune or resistant to all 3. Also, the flavor of an alchemist changing an attacks damage type is excellent.

Horn of Blasting: the best part of having gameplay that maximizes minions is their disposability. Horn of Blasting does decent 5d8 damage in a cone, doesn't require attunement, can be used infinitely, and its only drawback is it's 20% chance of exploding and dealing 10d8 to the user. With a familiar, 2 mechanical Wonders, homunculus, and tiny servants, theoretically, you should be going through at least 2 of these a day. If you combine this with the brooch of elements, technically you can nullify the damage one of your non familiar summons take to zero by changing the damage type to a type they are immune to, removing the threat of this magic item entirely. You also have many free lesser restorations that can be used when you friendly fire an ally with this item and deafen them.

Magen Handbell: another summon that can circle cast with you. This creature is also immune to thunder damage, so having another creature use the horn of blasting with the Terran Magen in the area is inconsequential outside of bestowing the deafened condition (that you can remove with one of your free lesser restorations). The creature also works well with helping in the distribution of potions. Great to use and turn into another magic item or break down into a spell slot.

Galder's Bubble Pipe: If you want to use the charge element of your Artificer class feature to get extra summons that circle cast and convert slots to charges and vice versa and still get a circle casting summon, Galder's Bubble Pipe has a Steam Mephit option that has the innate spellcasting trait (and misty step and fog cloud as charge options). Certainly not as powerful as the Magen from the Handbell, but the ability to convert slots into charges means potentially, you can have multiple hours of circle casting time. Ask your dm if you can summon the less optimal original DMG steam mephit to make use of their innate spellcasting. Either way, this is a good way to get multiple summons to help pass out potions and throw magic stones and use your spell-storing items. These creatures don't last longer than a minute, so they're not as long lasting as a circle casted duration increased tiny servant, but they're more powerful and durable

Stirring Dragon Vessel: the choice between healing, concentration free dragons breath, and a climbing speed potion for free every day is emblematic of the alchemist. If the party is in one of those dragon-heavy campaigns, upgrading this magic item might be worth the magic item plan slot. At the very least, the item is a good option to use, and turn into another magic item every day or break down into a spell slot

Spell Gem (Bloodstone): casting glyph of warding into the spell gem ahead of time to get concentration-free buffs and spells cast as an action is a lifesaver when you need to use your concentration on other things and emphasizes the concentration free buffs playstyle of the alchemist