Dual Status Alien F1 OPT to H1B SINGLE file

Hi,
Please read the situation below and advise how to file 2020 tax return. In 2020 my visa status got changed from F-1 OPT to H1B. 

• F1 Visa - 08/03/2018-09/30/2020 (789 Calendar Days / 2 yrs+1 month+ 27days)
• H1B Visa - 10/01/2020-12/31/2020 (91 Calendar Days)

IRS Official Link: Publication 519 Link: https://www.irs.gov/pub/irs-pdf/p519.pdf 

Step 1: Substantial Presence Test Calculation: Refer Page 4 of Publication 519
For TAX Year 2020:
1/6*0 days (2018) + 1/3*0 days (2019) + 91 days (2020) = 91 days
Thus, I will fail the test and I will be treated as non-resident.

For Year 2021:

1/6*0 days (2018) + 1/3*90 days (2020) + 153 days (2021) = 183 days

Result: I'll pass the test on June 3rd, 2021.  Correct? 

Step 2: DUAL STATUS ALIEN - Refer Page 7 of  Publication 519

Step 3: FIRST YEAR CHOICE - Refer Page 8 of  Publication 519

As I'll be able to pass the substantial test in 2021 and not in 2020, I'll have to opt in for "FIRST YEAR CHOICE" in a way to consider myself as resident alien for a part of 2020 year. Correct?

Step 4: For the Part of NON RESIDENT ALIEN ( Jan - Sept 2020). 
I have to consider all my US income forJan - Sept part only. Correct?

As I'm from India I can claim the Tax treaty for Student and take the standard deduction.Refer Page 28 of  Publication 519 (under Students and business apprentices from India).

As, I am NON RESIDENT ALIEN for only a part of year (Jan-Sept),  I can claim full standard deduction (due to treaty) for my US income (for part of year (Jan-Sept))  in my 1040 NR EZ form.  Correct?

Step 5: For the Part of RESIDENT ALIEN ( Oct - Dec 2020). 

I have to consider all my income for Oct - Dec part only.  Correct?

I am aware that I cannot claim standard deduction, but I can still claim itemized deduction. 

Step 6: How to File? Refer Page 34 of  Publication 519 

I will have to request extension to satisfy substantial presence test for year 2021. 

As I was a Resident at end of year, I will have to file 1040 and attach 1040-NR. 

Here is what I am confused about. 
As I have a single W-2 for the entire year. I have to split my income for part I was Resident Alien and part I was non resident alien and I will be taxed accordingly.  Correct?

Example: Say my Income on W2 for whole year is 100K ( This is my only income, no other income, no other w2, no outside US income). If I look at Payslips and do some math.

Pay for period Jan - Sept 2020 is 80K. (On F1 OPT, NON RESIDENT ALIEN) 
Pay for period Oct - Dec 2020 is 20K.  (On H1B, RESIDENT ALIEN) 

Step 6(a)
When filing my 1040-NR (as a statement to show my income for part of non resident alien) , I have to take only my 80K income and I will be taxed according to the US Tax brackets rate for my this income (80K) only. I can claim my deduction here as in Step 4.   Correct?

Step 6(b)
When filing my 1040, I have to take only my 20K income and I will be taxed according to the US Tax brackets rate for my this income (20K) only.  Correct?

Combining both, I can file my return for year 2020. Correct?

Conclusion - I'll be able to do single filing return using 1040 form by making the choice for - Dual Status Alien+First Year Choice.  Filing 1040 form  for my income for Oct- Dec and attaching 1040-NR (with deductions) for my income for Jan- Sept. Correct? 

Please advise. Thank you and really appreciate you for taking time.