This subclass is almost perfect for the character I want to play, but I think I want to homebrew the expanded spell list to customize the spells I'll be getting from cleric. I'm playing it as a drow, so daylight is an awful spell for my character and light is redundant with dancing lights... and there are some other classic cleric spells missing from the list overall. Here's the differences:

Cantrips: Light Guidance, Sacred Flame

1st: Cure Wounds; Guiding Bolt

2nd: Flaming Sphere Aid; Lesser Restoration Spiritual Weapon 

3rd: Revivify; Daylight Spirit Guardians 

4th: Guardian of Faith; Wall of Fire

5th: Greater Restoration; Flame Strike Mass Cure Wounds 

Also the 6th Level ability is pretty underwhelming because radiant damage is so rare. Would it be OP to make it radiant damage and fire damage from both the resistance and the cantrip buffs? 

Basically - Are these changes within the realm of reason? 

To be honest, I wouldn't allow this in my game. There is good reason the spell list doesn't include the spells you're adding, and the relative usefulness of radiant resistance is factored into the balance of the class.

In general I'm not opposed to tweaks, but every single one of these is geared towards making your character stronger, and I just don't think celestial warlock needs it.

Scatterbraind makes a good point, at the very least, I'd suggest replacing spiritual weapon with scorching ray, and mass cure wounds with banishing smite, and leaving radiant soul as is. But whatever the DM's cool with is really the way it goes.

Alright - nerfs it is. I think our next DM (I'm currently DMing our group) is pretty permissive on homebrew builds but I don't want to be taking too much advantage. So, how about:

Cantrips: Light Spare the Dying, Sacred Flame

1st: Cure Wounds; Guiding Bolt

2nd: Flaming Sphere; Lesser Restoration Aid

3rd: Revivify; Daylight Beacon of Hope

4th: Guardian of Faith; Wall of Fire

5th: Greater Restoration; Flame Strike ????

Cantrips: I really do want to swap out light because it's redundant, and wanted to do it with some sort of buff for other party members. Guidance was my first obvious thought, but yeah it's the best cantrip in the game. So let's swap that for spare the dying instead. Much more situational, but definitely useful when your party has no other options.  I'm very likely to go pact of the tome and take guidance anyway at level 3, but I get it. Don't be greedy lol.  

1st: Fine as is

2nd: I really wanted Spiritual Weapon because Celestial's offensive action economy is so limited as far as bonus actions are concerned. It's literally just Flaming Sphere which is comparable to Spiritual Weapon but a worse damage type and unfortunately requires concentration. I suppose that's intentional, but your right it's definitely an offensive bump on average. So I'll leave that one at flaming sphere. Is Aid too good on a Warlock? It scales really well, but doesn't stack on targets ... seems really useful so I'd love to keep that. 

3rd: I very much want to get rid of daylight since sunlight sensitivity is a thing for my character. Maybe Beacon of Hope instead of Spirit Guardians? It's not as much of a cleric staple so the novelty could be good, and it would work really well on a turn for Celestial Warlock since healing light is technically not a spell and can be cast as a bonus action meaning your could use both on one turn. 

4th: Fine as is

5th: I honestly don't love many of the level 5 cleric spells other than Greater Restoration and Commune ... but Commune feels a little off for a Warlock. Maybe Holy Weapon? I honestly didn't think Mass Cure wounds was a great spell unless a player's party/allies are numerous. 3d8+5 per creature healing is ... meh for a for 5th level slot. Seems comparable to aura of vitality which is two levels lower (though granted it's a slower burn of a spell most likely used after combat).  I appreciate the Banishing Smite suggestion, but I was trying to keep it to the cleric spell list.  

Those seem fine to me, and I'd be fine with keeping MCW at 5th too.

As for your bonus action, there's always hex. I think people underestimate how often you need to reapply it in most fights.

Looking better I'd say, Holy Weapon seems like a good choice as well. I'm a pretty lenient DM too and usually open to playtesting a work in progress as long as its understood there may be adjustments if something ends up being a little too good or too far into another player's lane. 

Long before the Celestial patron released, I homebrewed a few alternate 'force of good/light' warlock patrons, and I remember putting banishing smite on the list for some of them (as there wasn't so much of a spell selection back then). The official spell list ended up pretty similar to some of them. I had The Radiant, The Seraphim, and The Divine Arbiter, and my player went with the Radiant (a planetar in service to Amaunator) in our playtest.

I also had a Pact of the Corona, in what turned out to be an incredible case of unfortunate naming, in retrospect. Neat idea for a piece of headgear representing the pact, but sheesh it didn't age well. Beyond's homebrew tools still don't seem very good for making new pacts or invocations though.