What do you think about transferring my ETF (VT) from FSMOne to TD Ameritrade? Are there any cons that I could face?

There may be some fees with regards to foreign exchange.

Also, do consider just selling the positions and opening them up at TD Ameritrade. The commission fee for selling the positions in FSMOne is probably less than the transfer fee. TD Ameritrade has no commission fee anyway.

You wouldn’t lose any money if you buy and sell the positions at the same price.

Apart from the support issues that they currently face due to an overwhelming number of new user applications, I don't see much of a downside going with TD Ameritrade. After all, your use case is to buy and hold, so a broker with the least fees will be the most beneficial for you.

Do note that apart from a withdrawal fee, they also only support withdrawals in USD. Your bank may charge you fees as it will be an international wire transfer, and if you do not have a multi-currency account, you may incur foreign currency exchange fees (or poor exchange rates). Deposits may also take longer to process as compared to FSMOne. However, for your purposes, these should not be of much effect to you.