Welcome Nicola.
The answer to the first question is both yes and no. Yes, it is possible to pair bluetooth speakes to your R-Pi. The Pi 3+ has on-board bluetooth to do so. No, this is not natively supported by Volumio. There has been some development on a Bluetooth Speaker Plugin, but I’m not sure how that is working now. Last time I checked, it required a lot of manual configuration. Depending on your level of expertise with Linux, you may want to give this a try 
As for the Toslink option, this is an easy one. There are various hats (= expansion board that stacks on top of your Pi) available for a Toslink output. Examples are HifiBerry Digi+, JustBoom Digi Hat or the Allo DigiOne. Or you can even get cheap copies of these for 10-15€ from AliExpress.
The “official” ones, that is HifiBerry, JustBoom and Allo, are garantueed to work under Volumio. The Allo is quite a bit more expensive because it has its own on-board clock, for the real audiophiles. I guess that is overkill for what you aim to achieve.