A fighter using Booming Blade means not benefiting from its Extra Attacks. It’s nice at low level, but replacing 1 or 2 of your swings with some damage if you hit is situational. See this analysis. There are utility cantrips. For use in combat I’d pick a ranged damage spell and Mind Sliver is only Verbal so no need to free a hand up.

Have you considered prestidigitation or minor illusion?

They’re my two favorite spells on the game. Usually if I’m playing a spellcasting with access to them I’ll grab at least one, if not both. Message and Mending are some follow up favorites. Guidance is mechanically very powerful and spammable, assuming you can stay out of earshot of unassuming NPCs you want to haggle with.

Prestidigitation is basically a very small, very localized Wish spell and can do nearly anything if you have creativity. Cleaning and soiling is good for role play, preventing disease (if you all track that sort of stuff), warming and cooling is good for role play as well. Everyone loves their cold drinks to stay frosty after all. You can also summon real small trinkets that are smaller than the palm of your hand. Want some sunglasses for walking away from an explosion?

Minor illusion can do much the same, but in a larger area. I’ve found it to be a solid choice for distracting enemies based on the situation. Once, we were being accosted by cannibalistic fish men, and I used minor illusion to conjure a fish man’s severed head near the party to distract them for a round. You can also create (some) cover using minor illusion, so the ranger or rogue can shoot their bow or crossbow with ½ or ¾ cover pretty much anywhere. This could work if you too if you’re at a range. Approaching an enemy with ranged weapons and they’re more than your walking speed away?

Walk 30 feet toward them, creat a 5 foot tall boulder, and stick yourself behind it and potentially gain +2 to +5 AC or be untargetable unless they see through the illusion.

Edit: the cover will only be useful if you have a decent spellcasting modifier and/or the creatures you’re fighting don’t have blind/truesight/tremorsense. I can’t find the rules offhand, but I believe seeing through illusions requires an investigation check of 10 + your spellcasting modifier to see through. So if you’re using Intelligence to cast you may only have a DC of 9-11 as a fighter, and many passive investigations can see through the illusions.

Consider a non-combat cantrip. You are a fighter and generally have a more optimal solution in combat using your class features.

Maybe something that fits the theme of your class: Mage hand - fits the echo knight theme of making a spectral hand to grab light things or pull levers. Minor illusion - more illusions and distractions, or mock up a battlefield diagram to share your plan. Message - battlefield commander ability to silently share plans before an attack Mending - maintain your own gear Prestidigitation - cleanup after battle, cool your undershirt on a hot day in armor, warm your coffee at night, start the campfire, show people a palm sized replica of your target/enemy.

Pick something that encourages roleplay out of combat. Give yourself some reason to engage in the downtime action between fights. It will hopefully give you a richer experience.

Booming Blade’s damage scaling is the same as Toll the Dead, but with a smaller damage die.

However, since its effect depends on a weapon hit instead of the elf spellcasting ability (intelligence), I suspect it will proc more often than Toll the Dead for most melee Fighters. Seems like a good choice if you want to your cantrip to be for combat.

On the other hand, since a fighter has plenty of weapons for doing damage, you might consider picking a utility cantrip instead. How often you use it would depend on your play style and DM, of course, but in my experience, all it takes is a little imagination to get tremendous value from utility cantrips.

Here are someone else’s thoughts on the wizard/elf cantrips. Keep in mind that they’re probably considered in the context of a wizard casting them, rather than a fighter, but the points noted there are worth considering regardless.

Booming Blade or Green Flame Blade are the best for Fighter in combat since they use your weapon stats to attack. You could also choose something not for combat since you will already be very good at combat anyway, and fighters usually don’t have as much utility outside of fights. Good utility cantrips are Mage Hand, Mold Earth, Shape Water, Prestidigitation, Minor Illusion, and Mending. Those are in no particular order but Mold Earth and Shape Water are my personal favourites.

Out of curiosity, what was your original choice and why do you think it’s not a good fit anymore?