You could contact a finance manager at a dealer and ask if he's received information for leasing.
If nothing there, Google 2020 BRZ lease and expect similar numbers.
Residual for a 2020 appears to be around 56% for a 3 years with 10k-12k miles.
This is not exactly a friendly residual. I feel that could easily be 5% higher.
It's almost impossible to buy a 3 year old BRZ for $16k-$20k with 30k-36k miles.
It's also difficult to buy any clean Carfax, manual 2014-2020 BRZ with under 60k miles for less than $20k
Dealers are asking $17k+ for 2014-2016's with up to 80k miles.
Pay attention to money factor, some places mark it up for additional profit to over 0.003, which is 7.2%
You want to land closer to 0.002 or less (4.8%). This is usually negotiable to some extent. Lease interest rates are usually higher than purchase interest rates.
I'd consider buying. Finance it for 60 or 72 months. Put down taxes, registration, fees, and anywhere from 0-15%. You're payment will likely be similar and you'll likely be able to sell or trade it at any point after 2 years with significant equity instead of giving it back to Subaru and them making thousands on it at auction.
Example, 2022 Premium manual.
MSRP $28,955 - putting down taxes, registration, and fees, which vary by location.
Finance the entire amount of $28,955.
60 month loan at 3%, payment is ~$520/month, balance is $12,105 after 36 payments
72 month loan at 3.5%, payment is ~$446/month, the balance is $15,236 after 36 payments.
You should probably put down 5-10% of msrp to get the payment down and you could probably avoid GAP.
This what I'll be doing. Financing around $25k. So putting down $3955 plus CA taxes, registration, fees, etc.