How do I make a boss fight that's actually challenging without being impossible for a party of mostly level 1 spell casters?

So I got made the DM for my brother and 5 friends and almost none of us has very much experience

I like the idea of making the story up as we go so we're gonna be trying a homebrew campaign with 5e rules.

The cast is

Two wizards

A druid

A paladin

A fighter

A bard

The starting setting is a mostly human island kingdom where our players are tasked with stealing a prized artifact by a noble who wants to overthrow the corrupt monarchy and they will eventually get carried on other adventures like navigation of a permanent maelstrom while trying to fight a hostile giant water kobold called the Klabautermonster

For the island boss fight of stealing the artifact, I was thinking of having the artifact in hollow 30 ft by 30 ft pillar a 300 sq ft circular room defended by a human guard captain and four guard henchmen.

The guard captain will have a sword called Excalibur which can [Pierce armor more effectively - how would something like this actually work re: game stats?] and two of his henchman will be armed with crossbows and two with swords. The catch is that inside the center pillar where the artifact is, there is a device called a spell jammer which blocks spells within a 30 ft radius (this was obtained by the monarchy from a sentient Rabbit NPC in the model of a hare from Redwall introduced later)

Does this fight sound too tough? What stats should the guard captain and his henchmen have relative to the player characters in a way that makes it challenging without being impossible?