Smarty has worked quite well for me in Europe but if I was travelling more frequently then I'd pick up another SIM from another MVNO just for "redundancy" in case the backhaul to the UK is congested with SIM 1 for example.
This is a good point and I think the 12GB roaming limit needs to be considered as well.
Going back to the OP, the issue that you have is that there is only one network that provides a hefty roaming allowance and that is EE if you have their £25/mo Roam Abroad pass, or if you have one of their accounts that give you an inclusive extra - where you can select the Roam Abroad pass. EE have a fair use of 50GB on that pass (for accounts that have more than 50GB of data), so whilst it ticks the box for a reasonable amount of roaming data, it sounds like you need more.
My strategy would probably be get a UK SIM that suits your abilities, then use an eSim to make up the difference. But a quick check of the likes of Airalo and easySIM show that your costs could mount up if you are looking for a recent amount of data. If that ends up being too expensive, I'd find a EU based network that I could use, but that then becomes a little more complicated depending on how you distribute your time (i.e. you might run up against their roaming restrictions if you spend too much time away from the nation you signed up with).
All in all, there isn't an easy, clear option for what you are specifying. An easy life, would be going for one of the EE accounts with an inclusive extra - a quick check shows a £36/mo unlimited data/calls/SMS with the roam abroad inclusive extra (just keep in mind prices will go up in March by CPI+3.9%) or as others have suggested, Smarty (with 12GB of EU roaming) + eSim (which depending on your usage, could be more than the EE option).