SQL Server partition and index - sql

Question

I have a requirement to design a table that is going to have around 80 million records. I created a partition for every month using persisted column (if its wrong suggest me the best way). Please find below scripts that I used to create tables and partition and the query that's going to be used often. Only Insertion and deletion will be done on this table.
-- Create the Partition Function
CREATE PARTITION FUNCTION PF_Invoice_item (int)
AS RANGE LEFT FOR VALUES (1,2,3,4,5,6,7,8,9,10,11,12);
-- Create the Partition Scheme
CREATE PARTITION SCHEME PS_Invoice_item
AS PARTITION PF_Invoice_item ALL TO ([Primary]);
CREATE TABLE [Invoice]
(
[invoice_id] [bigint] NOT NULL,
[Invoice_Number] [varchar](255) NULL,
[Invoice_Date] [date] NULL,
[Invoice_Total] [numeric](18, 2) NULL,
[Outstanding_Balance] [decimal](18, 2) NULL,
CONSTRAINT [PK_Invoice_id] PRIMARY KEY CLUSTERED([invoice_id] ASC)
)
CREATE TABLE [InvoiceItem](
[invoice_item_id] [bigint] NOT NULL,
[invoice_id] [bigint] NOT NULL,
[invoice_Date] [date] NULL,
[make] [varchar](255) NULL,
[serial_number] [varchar](255) NULL,
[asset_id] [varchar](100) NULL,
[application] [varchar](255) NULL,
[customer] [varchar](255) NULL,
[ucid] [varchar](255) NULL,
[dcn] [varchar](255) NULL,
[dcn_name] [varchar](255) NULL,
[device_serial_number] [varchar](255) NULL,
[subscription_name] [varchar](255) NULL,
[product_name] [varchar](255) NULL,
[subscription_start_date] [date] NULL,
[subscription_end_date] [date] NULL,
[duration] [varchar](50) NULL,
[promo_name] [varchar](255) NULL,
[promo_end_date] [date] NULL,
[discount] [decimal](18, 2) NULL,
[tax] [decimal](18, 2) NULL,
[line_item_total] [decimal](18, 2) NULL,
[mth] AS (datepart(month,[invoice_date])) PERSISTED NOT NULL,**
[RELATED_PRODUCT_RATEPLAN_NAME] [varchar](250) NULL,
[SUB_TOTAL] [decimal](18, 2) NULL,
[BILLING_START_DATE] [date] NULL,`enter code here`
[BILLING_END_DATE] [date] NULL,
[SUBSCRIPTION_ID] [varchar](200) NULL,
[DEVICE_TYPE] [varchar](200) NULL,
[BASE_OR_PROMO] [varchar](200) NULL,
CONSTRAINT [PK_InvoiceItem_ID] PRIMARY KEY CLUSTERED ([invoice_item_id]
ASC,[mth] ASC))
ON PS_Invoice_item(mth);
GO
ALTER TABLE [InvoiceItem] WITH CHECK ADD CONSTRAINT [FK_Invoice_ID]
FOREIGN KEY([invoice_id])
REFERENCES [Invoice] ([invoice_id])
GO
I will be using below queries
select subscription_name,duration,start_date,end_date,promotion_name,
promotion_end_date,sub_total,discount,tax,line_item_total from InvoiceItem
lt inner join Invoice on lt.invoice_id=invoice.invoice_id where
invoice.invoice_number='' and lt.customer='' and lt.ucid='' lt.make='' and
lt.SERIAL_NUMBER='' and lt.dcn='' and lt.application=''
select customer,make,application from billing.AssetApplicationTotals
lineItem inner join billing.Invoice invoice on
lineItem.invoice_id=invoice.invoice_id where invoice.invoice_number='';
SELECT [invoice_Date],[make],[serial_number],[application],[customer],
[ucid],[dcn],[dcn_name],[device_serial_number]
,[subscription_name],[product_name],[subscription_start_date],
[subscription_end_date],[duration],[promo_name],[promo_end_date]
FROM [InvoiceItem] where [application]=''
SELECT [invoice_Date],[make],[serial_number],[application],[customer],
[ucid],[dcn],[dcn_name],[device_serial_number]
,[subscription_name],[product_name],[subscription_start_date],
[subscription_end_date],[duration],[promo_name],[promo_end_date]
FROM [InvoiceItem] where [customer]=''
What is the best way to create index? Shall I create separate non clustered index for each filter, or shall I have Composite index and shall I have covering index to avoid key lookup?