Have a shield spawn in with this mob

You have actually done most of the work.

Lets look specifically at the shield portion of the original command:

{id:"minecraft:shield",tag:{ench:[...]},Count:1}

We want to add BlockEntityTag data to that:

{id:"minecraft:shield",tag:{BlockEntityTag:{...},ench:[...]},Count:1}

You can copy and paste the BlockEntityTag data from the second command into the first command. Paste right after tag:{ and remember to add a comma after.

The command from doing that is:

/give @p spawn_egg 1 0 {EntityTag:{id:"minecraft:zombie",CustomName:"EMPERIAL SHADOW GATE GUARDS",CustomNameVisible:1,Health:999,Attributes:[{Name:"generic.followRange",Base:154},{Name:"generic.knockbackResistance",Base:1f},{Name:"generic.movementSpeed",Base:0.6f},{Name:"generic.attackDamage",Base:9999},{Name:"zombie.spawnReinforcements",Base:1f}],Silent:1,Invulnerable:1,CanBreakDoors:1,HandItems:[{id:"minecraft:diamond_sword",tag:{ench:[{id:2,lvl:9999},{id:0,lvl:9999},{id:1,lvl:9999},{id:3,lvl:9999},{id:4,lvl:9999},{id:7,lvl:9999},{id:9,lvl:9999},{id:16,lvl:9999},{id:17,lvl:9999},{id:19,lvl:9999},{id:20,lvl:9999},{id:34,lvl:9999},{id:71,lvl:9999},{id:16,lvl:999}]},Count:1},{id:"minecraft:shield",tag:{BlockEntityTag:{Base:0,Patterns:[{Color:1,Pattern:"cbo"},{Color:4,Pattern:"mr"},{Color:0,Pattern:"cbo"},{Color:0,Pattern:"bl"},{Color:0,Pattern:"tr"},{Color:0,Pattern:"mc"}]},ench:[{id:2,lvl:9999},{id:0,lvl:9999},{id:1,lvl:9999},{id:3,lvl:9999},{id:4,lvl:9999},{id:7,lvl:9999},{id:9,lvl:9999},{id:16,lvl:9999},{id:17,lvl:9999},{id:19,lvl:9999},{id:20,lvl:9999},{id:34,lvl:9999},{id:71,lvl:9999},{id:16,lvl:999}]},Count:1}],ArmorItems:[{id:"minecraft:diamond_boots",Count:1},{tag:{display:{Name:"shadow Imperial GATE GUARDS GEAR"},ench:[{id:2,lvl:9999},{id:0,lvl:9999},{id:1,lvl:9999},{id:3,lvl:9999},{id:4,lvl:9999},{id:7,lvl:9999},{id:9,lvl:9999},{id:34,lvl:9999},{id:71,lvl:9999}]},id:"minecraft:diamond_leggings",Count:1},{tag:{ench:[{id:2,lvl:9999},{id:0,lvl:9999},{id:1,lvl:9999},{id:3,lvl:9999},{id:4,lvl:9999},{id:7,lvl:9999},{id:9,lvl:9999},{id:34,lvl:9999},{id:71,lvl:9999}]},id:"minecraft:diamond_chestplate",Count:1},{id:"minecraft:diamond_helmet",Count:1,tag:{ench:[{id:2,lvl:9999},{id:0,lvl:9999},{id:1,lvl:9999},{id:3,lvl:9999},{id:4,lvl:9999},{id:7,lvl:9999},{id:9,lvl:9999},{id:34,lvl:9999},{id:71,lvl:9999}]}}],ActiveEffects:[{Id:3,Amplifier:127,Duration:2147483647},{Id:5,Amplifier:127,Duration:2147483647},{Id:10,Amplifier:127,Duration:2147483647},{Id:12,Amplifier:127,Duration:2147483647},{Id:13,Amplifier:127,Duration:2147483647},{Id:21,Amplifier:127,Duration:2147483647},{Id:22,Amplifier:127,Duration:2147483647}],Tags:["SHADOW EMPERIAL GATE GUARDS"],Team:Shadow}}

This gives an egg which yields this guy:

/summon minecraft:zombie ~ ~1 ~ {CustomName:""Tier 100 Zombie"",CustomNameVisible:1,Health:50,Attributes:[{Name:"generic.maxHealth",Base:50},{Name:"generic.followRange",Base:226},{Name:"generic.knockbackResistance",Base:0.23f},{Name:"generic.movementSpeed",Base:4.7f},{Name:"generic.attackDamage",Base:7},{Name:"zombie.spawnReinforcements",Base:0.61f}],IsBaby:0,HandItems:[{id:"minecraft:netherite_axe",tag:{display:{Name:'{"text":"the best weapon"}',Lore:['{"text":"not really lol"}']},Enchantments:[{id:sharpness,lvl:5}]},Count:1},{id:"minecraft:shield",tag:{display:{Name:'{"text":"yes"}',Lore:['{"text":"no"}']},Enchantments:[{id:unbreaking,lvl:3}]},Count:1}],HandDropChances:[0.47F,0F],ArmorItems:[{tag:{Enchantments:[{id:protection,lvl:4}]},id:"minecraft:diamond_boots",Count:1},{tag:{Enchantments:[{id:protection,lvl:4}]},id:"minecraft:diamond_leggings",Count:1},{tag:{Enchantments:[{id:protection,lvl:4}]},id:"minecraft:diamond_chestplate",Count:1},{id:"minecraft:diamond_helmet",Count:1,tag:{Enchantments:[{id:protection,lvl:4}]}}]}