Weekly “Help Me FIRE!” thread. Post your detailed information for highly specific advice - August 30, 2021

-Hey all! New around here and looking to retire ASAP (Ideally within the next 6 years) I'm hoping anyone who is bored and doesn't mind spending a few minutes giving advice would kindly let me know what they think of my situation :)
36 Year old male, ESL teacher in South Korea but from the USA
My goal is retire at 40 with 1 mil USD and live a somewhat frugal but comfortable life. I hope at 1 mil my investments will support myself and leave enough left over that it can still grow.
I work as an ESL teacher in South Korea. I work at a private school but fortunately "full-time" at my job is only about 25 hours a week, leaving me lots of time to do private tutoring and seek out other employment as I have a long-term resident visa.
My income varies from month to month. My actual job is not that high at about 2,000 USD after taxes, however, I can earn around 7,500 USD during my busy months. Summer and Winter are offseasons when my side jobs take vacations. So far this year I have saved about 28k USD and should have saved about 36k after all expenses and taxes. Savings of 35k-40k per year should be the standard for the next 6 years.
In terms of my budget, I live in an one room apartment. I have a motorcycle which I use for transportation as it allows me to get around the city faster than public transportation. It's been a great asset for me to find side jobs and get to them quickly, letting me pick up more jobs than when I didn't have a car. All in all, I spend around 1.8k-2k USD per month on living expenses.

My assets:
4 rental properties

Home 1: 50k / I owe 13k - Rents for 500 USD a month.
Home 2: 185,000 (possibly 200k if I'm lucky) / I owe 65k - Rents for 1,550 a month. (It's a duplex and one side pays vastly under market value because they were already living there and I got the house for only 135k. I let them stay at the rent they were paying. I am inclined to raise it soon, but feel terrible at the same time because they are approaching 75 years old and have zero savings. The wife is still working minimum wage jobs just to make it)

Home 3: 165,000k / I owe 65k - Rents for 1,400 a month.

Home 4: 130k/ I owe 100k- Rents for 1,000 a month.

Stock Market: 71k in VOO

Crypto: 5,500 USD (60% Ethereum / 20% Bitcoin / 20% BNB) I have an RTX 3090 and an RTX 3080 (graphic cards) that I am mining Ethereum with. It fluctuates, but around $12-17 a day is average.

Korean pension: 30k. In 6 years it should be worth about 45k and if I leave the country and cancel my visa, I receive this in a lump sum.

Debt: only the mortgages on my houses. $243,000.

Home 1: 5 years remaining on mortgage. I owe 13k. Monthly payment is 320 USD

Home 2: 8 years 9 months remaining. I owe 65k. Monthly payment about 650 USD.

Home 3: 8 Years remaining. I owe 65k. Monthly payment about 650USD.

Home 4: 29.5 years remaining. I owe 100k. Monthly payment 650 USD.

This puts my monthly income from rentals $4,450 - 2,270 = Around 2,180.
Health concerns: I have had terrible joints for about 15 years now. I used steroids as a teen and in university (that's another story) and basically destroyed my shoulder and hip labrums. I've had several surgeries to reattach them but they keep tearing so I would really like to retire by 40 and have to worry about taking time off from work if I need an operation or just hurt and don't feel like going to work. The medical system is quite cheap in Korea, so medical expenses aren't high, but time off due to surgeries has killed my income at times. I generally try to time to surgeries during my vacation to minimize the impact on my income.
Family: Currently single with no plans to have children. I may get marries one day, but I have no desire to have kids. My parents are very secure in their finances and I expect to receive an inheritance of 200-300k from them easily and they won't need any financial help from me. However, they are only 25 years older than me, so I will be will into retirement before I would expect that. I also enjoy having them around so that's a good thing ;-)
Quick overview of finances:

363,500 USD in assets right now after debt, expected 45k when I give up my visa 6 years later.

Average 35k-40k a year in after tax income that can be invested. Average around 2k a month that can be used from my rentals.
What I want to know: Would it be best to keep money in VOO or take that money and pay off House 1, and nearly all of House 2? Then use that month to work towards paying off House 3 within the next 2 years? Then take all of that income plus my income at work at pay of House 4 within 4 years? As it stands now, house 1 will pay itself off before my ideal retirement date. The other houses will need extra income put in to pay them off before their maturity date.

At this point, I am investing all my spare money into VOO each month. Should I stick to real estate and pay off my houses now or even expand my rental inventory and try to pick up one or two more? There are a few houses for sale around 100-125k that I'm sure will rent for 1,000 a month.

Is retiring at 40 or 41 with around 1mil within my reach? If I earn an average of 10% on my investments, and continue to invest around 2.8k a month, finance calculator say yes. But I am I being too optimistic with that return rate?

Any comments or suggestions are VERY much appreciated. Thanks a lot!

/r/financialindependence Thread