I don't think it worth reserving seats. If you don't reserve, you can choose from what is available when you check in, and there is no charge. If you check in online the day before the flight, you will probably have a generous choice of seats.

We have flown Ryanair a few times with no problem. We did fly from Venice to Dublin on Ryanair. It is a cheap price but you are limited to the amount of luggage you take or you have to pay an additional fee.

Ryanir does not fly into Venice - you fly into a small town which is about one hour more or less away. So you do have a bus ticket to pay for going from the airport to the bus "station" in Venice.

Also, we changed planes at Stanstead Airport in London.

I think you are better off to fly with Aer Lingus if it is a direct flight and lands in Venice.