How do I handle magic items in this situation?

[Sir-Carlos]

Okay, I am DMing a campaign. We started with a one-shot that was pre-written and the loot was a +1 Sword and a wand of secrets and some money. One of the players dropped out after the one shot and the new one that joined, asked me if he gets any additional equipment. That was the first time I wondered about loot. Of 4 players, 2 now have magic items. and the +1 sword is far betetr than the wand of secrets. We will play a pre-written adventure next and there is a long time without many additional magic items. There will be, but only later. One plot-relevant item will come up soon, but idk if that should coun`t against the items they already have or not. What do I do? They havent identified the wand yet, so maybe I switch it for something more useful? (There arent a lot of secret doors in the campaign)

But I don`t know if I should give them more magic items I don`t feel confident to give them out willy-nilly and potentially derailing the campaign. What do I do? they are level 3, currently. My current plan is to just switch the wand for a more useful one and jsut give everyone a small magic item. any ideas as of which ones I should give them? Or should I just not sweat it and continue without overthinking?

[Mastikator]

Put a magic item that is useful to the other two players in the hands of their enemies. I.e yes give them something, but let them fight for it.

[J-H]

Use the list over at Donjon and come up with some less powerful magic items if you want. A moon-touched sword is magic and glows, but doesn't do +to hit/damage.
A Wizard's hat lets you try to cast any cantrip 1/day.
etc.

[Sorinth]

I don't think you have to worry too much about derailing a campaign if you hand out generic/utility magic items. For instance an immovable rod can be a lot of fun in the right hands and can make for some fun shenanigans but it's unlikely to trivialize an adventure. A bag of holding is very convenient but again not going to change much overall. Also common magic items from I think Xanathar's were designed to give low level players something magical that won't break anything. That said at level 3 there's no pressure to hand out magic loot.

It's worth noting you can always ask what the players want and then decide from there whether it will cause problems. Knowing what they want also gives you the opportunity to hand out the recipe instead of the item, this gives them something to work towards and you can swap out treasure from your next adventure for reagents to make the magic items.

[Reach Weapon]

Of 4 players, 2 now have magic items. and the +1 sword is far betetr than the wand of secrets. [...] But I don`t know if I should give them more magic items I don`t feel confident to give them out willy-nilly and potentially derailing the campaign. What do I do?

In general, I believe that the DM shouldn't involve themselves in party dynamics (although they should often take the lead in table issues), so my knee-jerk opinion is stay out of it, don't change course.

That said, given the changes to your table, I'd consider having a suitable NPC offer to trade them some lesser magic items for the +1 sword (perhaps as the agent of someone else).

[Sir-Carlos]

Put a magic item that is useful to the other two players in the hands of their enemies. I.e yes give them something, but let them fight for it.

That is a good idea. should I swap the wand of secrets? I thought of maybe making it a wand of magic missile.

[da newt]

If I was the DM, I'd have a wierdo roll up with a wagon full of very low level magic items and give the party a chance to barter. Maybe they want to exchange the +1 sword for 2 moon touched weapons, and the wand of secrets (a fairly lack luster magic item in my opinion) for 2 things the others would like to have that are pretty mundane and or interesting (if you do want to go w/ a wand of MM, I'd decrease the charges to ~ 3 per day).

For early campaigns I like to drop stuff other than just +1 weapons - things that will allow the players to do different things (hat of disguise, tentacle rod, bag of beans, bag of tricks, circlet of blasting, deck of illusions, onyx dog, etc), but that's personal preference.

Then as you proceed you can offer your players chances to go on side quests to earn fun items or to follow the written adventure. This way they have some agency, and can earn rewards.

[Mastikator]

That is a good idea. should I swap the wand of secrets? I thought of maybe making it a wand of magic missile.

Wand of the war mage, wand of magic detection, even wand of web. You could even homebrew a magic item (DMG chapter 7: random magic -> minor property for inspiration), or just enable the wielder to cast a single cantrip using the wand.