I just want to make sure I will get USD 23.000 for each btc I receive around September 2021.

I don't want to speculate with this money.
If you buy a put option, you have the right to sell crypto at X date for Y USD. That could be used to speculate or to reduce risk (my case).
I can not ask to be paid in fiat, the payment will be a fixed amount of btc.

I am going to do some work and I will get paid a fixed amount of BTC in around 9 months.
I plan to sell those BTC as I receive them.
I am happy with the current BTC/USD market price and I don't want to speculate.
Is there a trade I could do to compensate for the BTC/USD volatility?

I don't have the amount of BTC I will eventually receive, so I can not sell my own BTC now as a hedge.
I guess the de-facto way to do this is to buy a put option. AFAIK deribit.com is the only exchange I could use to do this.
Short sell with margin trade on a bigger exchange is a valid option. But I may get out of collateral if BTC/USD goes up and down several times in the next few months.
I would prefer to minimize having any kind of funds on exchanges (especially smaller ones) because they are usually hacked or go bankrupt. If having funds in a crypto exchange is required, I would try to split the trade between several exchanges.

So, what would be the lesser evil?

I just published https://github.com/electrumj/electrumj

It is a low-level client for ElectrumX. I plan to add higher-level
code (wallet, blockchain, server discovery, etc) in the future.

What bitcoin testnet explorer do you use?

I used to use https://www.blocktrail.com/tBTC
I like the interface but I saw today sometimes the information is not up to date.

An alternative is https://tbtc.blockr.io
Information seems to be up to date.
Some information is not shown eg. block relay time.
There is no javascript checking constantly for new information, you have to reload the page.