Your post indicates that your basic issue with forex trading through a standard forex broker is the cost of swaps, when positions are held for long periods of time.
As a swing trader, whose trades are typically held for weeks, and are often held for months, I have faced that issue. My solution was to find a reputable broker that offers swap-free accounts to all traders (not just those professing Islamic religious objections to the charging of interest).
One such broker is FxGlory, domiciled in St. Vincent and the Grenadines (in the Caribbean region). In our thread Going offshore to escape the CFTC, we vetted FxGlory over 8 years ago. Since 2012, they have been on the List of brokers we consider to be reputable and worthy of consideration by readers of our thread. Today, FxGlory is one of 12 brokers on that List.
There are other brokers, in various countries around the world, that offer swap-free forex accounts to all clients. If FxGlory doesn’t meet your needs, you’ll have to do some research to find alternatives.
You haven’t told us where you are located. But, you referred to depositing large sums of euro into bank savings accounts, which leads me to guess that you are in the EU. If you start your search for a swap-free forex broker regulated in the EU, you might come up short, and find that you have to look offshore. Offshore is not for everyone. But, if you research it thoroughly, you might find that it’s the solution to your problem.
Here is another issue, which is larger than the swap versus swap-free consideration.
Trading foreign currencies unleveraged – which is what you are proposing to do with multiple savings accounts – is a very inefficient form of speculation. You gave the example of converting €200,000 into dollars, and then some months later converting it back. There are three things to consider, right from the start:
In order to play that game, you have to tie up €200,000 of real money for the duration of your “trade”, because you are trading without the benefit of leverage. In a forex account, by contrast, this trade would amount to two standard lots of EUR/USD, requiring margin of less than €10,000 total (depending on your broker’s required initial margin).
If you have hundreds of thousands of euro available for currency trading, you can do much better than a two-lot speculation. Trading with leverage through a forex broker will allow you to trade a notional amount which is many times the size of your trading capital.
Second-tier and third-tier commercial banks which offer the sort of currency transactions you are describing typically charge wider spreads to buy and sell currencies than do forex brokers. Spreads — even ridiculously large spreads — are a small cost, compared to the erosion of capital resulting from negative swaps in long-term trades. But, spreads are a cost, nevertheless, and you should factor them into your comparison of the savings-account idea versus a standard forex account.