ERC20 detucts a lot of fee but TRC 20 on tron network, the transaction is free of cost.
I want to know if TRC 20 blockchain is safe to use and we can store our USDT there for long term ?
No need to store your USDT there for long term just for transaction cost. It is safe to transfer your USDT to TRC 20 (Tron) blockchain. The cost of TRC-20 USDT transaction is very low and can be withdrawn completely free from some exchanges like Binance, Kucoin etc. I often withdraw my asset by converting it to TRC-20 USDT.
it is actually the best way to transfer USDT from one exchange to another, or from a wallet to another wallet, fast and low cost transfer
with all said, I would not stuck many USDT on Tron network, it is not decentralized and security issues are not negligible there, so keep it fast and clean, with transfer to Ethereum network for security in the end