I upgraded to a smart trainer 2 years ago after a couple decades of using various dumb trainers.
A smart trainer fundamentally changes the Zwift experience because of sim mode so one is kind of a must have if you’re a dedicated Zwiftie.
For Trainer Road, (or any other guided interval workout like on a Garmin, Zwift, etc) a smart trainer gets you erg mode and accurate power. While many sing the praises of erg mode, many others don’t regard erg mode as all that great and some very reputable coaches recommend against using it even if you have it available. Personally, I did not get along well with erg mode and have stopped using it for interval work. That leaves accurate power but you can get a power meter for quite a bit less than a smart trainer and have power data for outdoor rides too. A good dumb trainer (which you have) plus a power meter is the way to go if you are on a budget.
Smart trainers are awesome but its a want item not a need item. I’d put a power meter in the need category.