You are right about SUM, but not about VAT, in terms of being “way more complicated” than before.
For VAT you just need to install the VAT profile and select it for your item, and your item will be persisted with VAT exactly as before.
For SUM, you need to create a group with your items as child items, which in itself is more complicated than just adding a fixed amount in the configuration. However, it is also more flexible and allows you to add dynamic tariffs and other price components (even from other bindings) as well. You can still persist your group item and it will work as before, but unfortunately time series are not supported, so you cannot visualize future total prices out-of-the-box:
A work-around for that is admittedly way more complicated:
