Optimize a druid for debuff and healing

I won't build the entire character for you, but I can make a couple of recommendations to improve the druid to excel at what you are wanting to do.

The standard druid spell list will have you mostly covered, but since your DM is permitting small borrowing from other spell lists, I would recommend suggesting your DM allow you to take some of the following spells.

Healing and Buff Spells

Bless (1st level spell):

You bless up to three creatures of your choice within range. Whenever a target makes an attack roll or a saving throw before the spell ends, the target can roll a d4 and add the number rolled to the attack roll or saving throw.

Prayer of Healing (2nd level spell):

Up to six creatures of your choice that you can see within range each regain hit points equal to 2d8 + your spellcasting ability modifier. This spell has no effect on undead or constructs.

Beacon of Hope (3rd level spell):

This spell bestows hope and vitality. Choose any number of creatures within range. For the duration, each target has advantage on Wisdom saving throws and death saving throws, and regains the maximum number of hit points possible from any healing.

Debuff Spells

Bane (1st level spell):

Up to three creatures of your choice that you can see within range must make Charisma saving throws. Whenever a target that fails this saving throw makes an attack roll or a saving throw before the spell ends, the target must roll a d4 and subtract the number rolled from the attack roll or saving throw.

Blindness/Deafness (2nd level spell):

You can blind or deafen a foe. Choose one creature that you can see within range to make a Constitution saving throw. If it fails, the target is either blinded or deafened (your choice) for the duration. At the end of each of its turns, the target can make a Constitution saving throw. On a success, the spell ends.

Bestow Curse (3rd level spell):

You touch a creature, and that creature must succeed on a Wisdom saving throw or become cursed for the duration of the spell. When you cast this spell, choose the nature of the curse from the following options:

• Choose one ability score. While cursed, the target has disadvantage on ability checks and saving throws made with that ability score.
• While cursed, the target has disadvantage on attack rolls against you.
• While cursed, the target must make a Wisdom saving throw at the start of each of its turns. If it fails, it wastes its action that turn doing nothing.
• While the target is cursed, your attacks and spells deal an extra 1d8 necrotic damage to the target.

A remove curse spell ends this effect. At the DM's option, you may choose an alternative curse effect, but it should be no more powerful than those described above. The DM has final say on such a curse's effect.

These give you a couple more options to round out the druid's already decent support, healing, and debuff options. Your DM said "small editing", so maybe suggest taking one spell at each slot level, 1st-3rd, choosing between your favorite of the buff and debuff options listed at each slot level.

Balance

The Dungeon Master's Guide has this to say about changing spell lists:

Modifying a class’s spell list usually has little effect on a character’s power but can change the flavor of a class significantly. In your world, paladins might not swear their oaths to ideals, but instead swear fealty to powerful sorcerers. To capture this story concept, you could build a new paladin spell list with spells meant to protect their masters, drawn from the sorcerer or wizard lists. Suddenly, the paladin feels like a different class.

The main concern listed in the DMG is not affecting balance, but rather changing the flavor of a class, but that seems to be exactly what you are going for.

An Alternative to Druid: the Nature Cleric

The Cleric nature domain is described in this way:

Gods of nature are as varied as the natural world itself, from inscrutable gods of the deep forests (such as Silvanus, Obad-Hai, Chislev, Balinor, and Pan) to friendly deities associated with particular springs and groves (such as Eldath). Druids revere nature as a whole and might serve one of these deities, practicing mysterious rites and reciting all-but-forgotten prayers in their own secret tongue. But many of these gods have clerics as well, champions who take a more active role in advancing the interests of a particular nature god. These clerics might hunt the evil monstrosities that despoil the woodlands, bless the harvest of the faithful, or wither the crops of those who anger their gods.

Playing a nature domain cleric would give you full access to the cleric spell list, as well many abilities that are similar in flavor to the druid. Of course, you lose out on the abilities and spells that are entirely unique to the druid, most notably, wild shape. But this is another option, and one that is optimally suited for the support and debuff style of play you're looking for, and very similarly flavored from a roleplay perspective.

The core druid spells ought to have you covered for this.

By level five you could take:

CANTRIPS

• Guidance
• Resistance
• Shillelagh / Produce Flame / Primal Savagery

1ST LEVEL

• Faerie Fire
• Goodberry
• Healing Word
• Cure Wounds

2ND LEVEL

• Lesser Restoration
• Healing Spirit
• Moonbeam

3RD LEVEL

• Wind Wall or Wall of Water
• Call Lightening or Protection from Energy

This is purely an example of what you can do. Between the spell lists in the Player's Handbook and Xanathar's Guide to Everything, there are plenty of options within the standard Druid to create a good support character.

Don't forget that Druids can change their spell lists after a long rest, with the exception of Cantrips (although I've known DMs to allow that on a case-by-base basis). So you won't be locked-in, if you want to make changes down the road.