You will pay no CGT if you sell your old house within 12 months of it ceasing to be your Principal private residence.
After that, the gain is prorated. Let's say it takes 2 years to sell. and yo make a €200k gain. You owned it for 8 years. For 25% of the time, it was not you Principal private residence. So you pay CGT on 25% of €200k. Your taxable gain will be €50k. Your CGT will be €10k. (20% of 50)
The biggest issue for you will not be CGT, but paying the mortgage on an empty house. €350k@5% = €17k per year. Quite a few people have been caught out badly by buying before they sell. They have been unable to sell their house. They have been under severe financial pressure and so have been forced to sell the old house at any price.
Brendan