Can I use my credit card if it's not a US bank?

It is extremely risky. Even if your card is not US-based, the POS terminals in Cuba are often offline or incompatible. Always assume your cards will not work.

Is it safe to carry that much cash?

Cuba is generally very safe regarding violent crime. Keep your cash in a secure money belt under your clothes and never leave it unattended in your luggage.

Managing Money and Currency in Cuba

Carry enough cash in EUR or USD to cover your entire trip, as foreign bank cards do not work in Cuba. Exchange money on the informal market rather than at the official state rate to get a fair value for your currency.

Bring hard cash. Bring the entirety of your trip budget in physical banknotes (EUR or USD). US-issued credit cards and debit cards are 100% useless in Cuba due to the embargo, and even non-US cards rarely work at ATMs.
Avoid the official exchange rate. Do not exchange money at the airport or state-run banks (CADECA). You will receive an artificially low rate. Ask your Casa Particular host or restaurant owners for the 'informal' exchange rate, which is significantly better.
Keep denominations small. Avoid bringing $100 bills if possible. Many locals struggle to provide change for large notes. Bring a mix of $5, $10, and $20 bills to make transactions easier.
Pay for services in foreign currency. Many private businesses (Casa Particulars and paladares) prefer being paid directly in USD or EUR. Confirm your room rate in foreign currency before you arrive to avoid confusion.

Can I use my credit card if it's not a US bank?: It is extremely risky. Even if your card is not US-based, the POS terminals in Cuba are often offline or incompatible. Always assume your cards will not work.
Is it safe to carry that much cash?: Cuba is generally very safe regarding violent crime. Keep your cash in a secure money belt under your clothes and never leave it unattended in your luggage.