How would you rate my duplex purchase from 2019?

[Stephen G.]

Purchased in a desirable part of Oakland (Glenview area) as an owner occupant in 2019. Put 20% down @ 4.125% and refi'd in 2020 to 3.25% so my PITI is around $7k/month. My gross income is a bit less than $7k (and I'm renting near top of market rates) with minimal repairs ($2k/y), but...

1. Property has appreciated 10-15% since purchase.

2. Principal paydown is now around $2k/month

Tenants are great. No evictions, no turnover, and have not raised rents (honestly feel like I'm near the upper end already). 

Assuming the Oakland rental market continues to remain stagnant/competitive with newer buildings going up, is property appreciation the only way to increasing wealth with my building? In retrospect, did I overpay or make a mistake with the purchase? If I threw the money into SPY, it would be a 104% gain compared to walking away with an 80% gain after fees/commissions. I don't plan on selling; I'm mainly just curious how people make $ in these markets when rental rates are on the decline since COVID.

[Richard H.]

Quote from @Stephen G.:

Purchased in a desirable part of Oakland (Glenview area) as an owner occupant in 2019. Put 20% down @ 4.125% and refi'd in 2020 to 3.25% so my PITI is around $7k/month. My gross income is a bit less than $7k (and I'm renting near top of market rates) with minimal repairs ($2k/y), but...

1. Property has appreciated 10-15% since purchase.

2. Principal paydown is now around $2k/month

Tenants are great. No evictions, no turnover, and have not raised rents (honestly feel like I'm near the upper end already). 

Assuming the Oakland rental market continues to remain stagnant/competitive with newer buildings going up, is property appreciation the only way to increasing wealth with my building? In retrospect, did I overpay or make a mistake with the purchase? If I threw the money into SPY, it would be a 104% gain compared to walking away with an 80% gain after fees/commissions. I don't plan on selling; I'm mainly just curious how people make $ in these markets when rental rates are on the decline since COVID.

Is the 7k a month with both sides rented? If it’s in a great part of town, these generally tend to not be the best from a cash flow perspective. 
In terms of your question about appreciation being the only way to gain here, perhaps it is. Your other levers are revenue (which as you mentioned is capped) and expenses (which congrats to you are low). If you want more cash flow and want to walk away with more of the 80%, perhaps identify another market and do a 1031 exchange so as to not pay capital gains taxes.