1) Sounds to me like the immediate 15% ROI of the company stock beats all other options. But that answer depends a lot on long vesting requirements, fees, etc. If you could sell anytime, it would be a no-brainer. But I know there's a catch. Check the fine print. A 15% head start more than makes up for diversification, up to a point. Put in the entire 10% of pay if making a quick 15% is that easy!
2) My 2nd recommendation is the 401(k). It sounds like you are in a high-ish tax bracket, so deferring taxes to the post-FI phase of your life makes sense. It's like borrowing hundreds of thousands Also, it's a fast way to reach a predetermined FIRE number and build a savings snowball, even if it's a pre-tax savings snowball. Plus you have options to withdraw from these accounts without fines. Look up how to do a Roth conversion ladder. You actually don't need as much post-tax savings as you might think. Plow into your 401k up to the limit.
3) The real estate partnership seems to be doing well as a side gig, but as it grows it will require more of your time in one way or another. Will you be able to maintain your apparently well-paying job while doubling or tripling the size of the side gig? It's something to consider.
4) Ask if you can put 100% of your bonus in your 401(k). Tax withholdings are usually higher on bonuses, so you'd be flipping from paying a crapton of taxes you'll only partially recover with a big refund check a year later to making a huge tax-exempt investment ASAP. Double win.
5) I wouldn't bother changing the mortgage to juice your savings rate. It's a matter of taking money from the left pocket and putting it in the right pocket. Unless, that is, you want to leverage your house to pursue riskier investments. There's a lot of low-hanging fruit before you reach the point of risk arbitrage.
6) The investment order sticky thread covers what to do if you still have savings left over after maxing the ESOP, the 401(k), and then your Roth IRA in that order. My comments address your unique circumstances, such as the ESOP program and RE partnership. VTI is a fine investment choice. Just be sure to set dividends to reinvest!