Cant file as married separately as it requires ITIN as well

You must file a dual status return or make an election to file a married filing jointly full year resident return.  As you want to wait to amend your return to file jointly at a later date so you can apply for an ITIN, you must file a dual status return now.  When you do amend your tax return to apply for an ITIN, you must elect to file a jointly filed tax return.  You can find this election on page 9 of Publication 519 (link is below).

Dual status return (part year nonresident and part year resident) is filed in the first year or last year you are in the US (unless you elect to file a joint return with your spouse).  Filing a dual status return is a bit complicated.

 If you file a dual status return, you can only use the filing status of married filing separately (if you are married) or single.  Also, you are not allowed to take the standard deduction. 

In order to prepare a dual status return in TurboTax, you will need to print a copy of the nonresident return (Form 1040NR from the IRS website) for your tax on US income before arriving in the u There will most likely be no US source income to enter for that time.  Nonresidents are only taxed on US source income.  If you did have US source income during your nonresident part of 2018, please let me know as you will need to complete a nonresident return with Sprintax.  Write on the top of the 1st page of the 1040NR, Dual Statement. You will need to write in your name and address and input zero as your US source income (if that is the case). Enter your filing status (single or married filing separately).

https://www.irs.gov/pub/irs-pdf/f1040nr.pdf

Then for the resident part of the return you prepare in TurboTax, you are not allowed to use the standard deduction, only itemized deductions if you are filing a dual status return.  Also, you can either file single (if you are single) or married filing separately. You will need to enter a dummy SSN for your spouse in order to complete your return in TurboTax.  You can enter 999-88-9999 as a "dummy" number.  After you complete your return, you must print the return and "white-out" the dummy SSN and enter "NRA" (nonresident alien) in the space for the SSN.  You cannot e-file your return.

You must suppress the standard deduction.  Instructions for this are below.

When your resident part of the return is finished in TurboTax, print the return and write on top of the return, Dual Return. 

Put the 1040 on top of 1040NR nonresident return and staple and mail to the IRS.  You must sign the 1040 before mailing.  You do not need to sign the 1040NR statement.

You will need to staple your 1040NR and 1040 together and mail to the following address:

If you  are not enclosing a payment then use this address...

Department of the Treasury

Internal Revenue Service

Austin, TX  73301-0215

USA

If you are enclosing a payment then use this address...

Internal Revenue Service

P. O. Box 1303

Charlotte, NC 28201-1303

USA

Please see more help on dual status returns see page 7 and page 32 of Pub 519.

Publication 519

You must choose itemized deductions.  Please follow the below instructions to switch to itemized deductions.

TurboTax Online instructions for how to change from standard deduction to

First, sign in and make sure your return is open. You should see 2018 TAXES in your left-hand menu as pictured below (if not, tap or click the upper-left corner to expand the menu or select Take me to my return).

Once your return is open:

1. From the left-hand menu, select Federal (on mobile devices, you might need to scroll down).
2. Now select Deductions & Credits near the top of your screen.
3. When you get to Now it's time for the fun part... Your tax breaks, scroll all the way to the bottom and select Wrap up tax breaks (if you don't see this button, select Skip to see all tax breaks, then scroll down again).
4. Continue through the interview until you see The Standard [or Itemized] Deduction is Right for You, pictured below.
5. On that screen, check the box Change my deduction to see the dollar amount for each deduction.
6. Select Continue if you want to save any changes. If you're switching to the itemized deduction, you may be prompted to upgrade.

You will need to answer questions in TurboTax as to whether you had health insurance for every month.  You need to check that you had an exemption as you Spent 330 days or more outside the US.

If you file a dual status return, you will need to also check that exemption that you Spent 330 days or more outside the US to avoid the health insurance penalty.