No, the one attack is part of the spell that you cast using a Magic Action.

This means you can't use extra attack or two weapon-fighting because you can only use those if you used the Attack Action.

1. Be an Oath of Redemption paladin

2. Take the Magic Initiate origin feat to get True Strike, take Great Weapon Fighting as your fighting style, and take the War Mage feat

3. Acquire a 2d6 damage Vicious Weapon (such as a maul or greatsword) and slap a Ruby of the War Mage on it

4. Use Hold Person (or Hold Monster if necessary) on your target, then smack them with your maul and use Banishing Smite, instantly and unavoidably doing a total of 14d6+2d8+10d10+5 damage for an average of 118 damage without even taking into account the benefit of Great Weapon Fighting. Congratulations, you have achieved the smitiest smite there ever was! And here's the best part: depending on which version of Hold Person/Monster you had to use and whether your opponent breaks free or not, you can do it next turn too

Is there anything stopping me from using this with a crossbow? or a firearm?

Quote from GethHelder >>

Is there anything stopping me from using this with a crossbow? or a firearm?

The only requirements for the weapon are:

1. You have to be proficient with it
2. It has to be worth at least 1 copper piece

You can use it with any weapon, including a crossbow or firearm, as long as you meet those requirements.

This spell is not well program in Beyond Sheet, the spell should never add the spellcasting modifier to the damage cuz it is already calculate in the weapon damage.
For a character level 5 :
Attack roll (D20 + spellcasting modifier) 
Damage roll 1d6 (quarterstaff) + Spellcasting modifier (Bludgeonning or Radiant damage) + 1d6 (radiant damage).

The True Strike spell show : 1d6 + spellcasting modifier (this is wrong cuz the spell damage dont have the spellcasting modifier it's bound to the weapon attack).
It needs to be fix.

Actually that's wrong. Read the wording: "you make one attack with the weapon used in the spell’s casting". Specifically, "used in the spell's casting". In other words, you are making an attack using the spell, not making an attack separate from the spell. It's the same as Booming and Green-Flame Blade, they are spells that include weapon attacks in the casting.

Since Radiant Soul requires, as you say, "that it be a 'spell you cast [that] deals Radiant...damage' and requires that damage to be inflicted against 'against one of the spell’s targets'", and True Strike is a spell that can deal Radiant Damage, it holds that when dealing Radiant damage using True Strike, the Celestial Warlock's Radiant Soul class feature would be applied.

It also makes no sense why Agonizing Blast would NOT apply with True Strike because, again, the cantrip is dealing the damage through the spell. Even though you word it as "your weapon attack is dealing *extra* damage", it's still THROUGH the cantrip (and the base cantrip doesn't deal "extra" damage at all, it's base weapon damage, you just choose Radiant or the weapon's normal damage type, but either way it's magical since it's through a spell).
Agonizing Blast states, "You can apply your Charisma modifier to any damage-dealing Warlock cantrip", True Strike is a damage-dealing cantrip. No reason why it doesn't apply and any DM that says it doesn't is an idiot.

I'm surprised no one pointed this out in almost a year.

Because it's not correct.