is it worth it?

If you buy Eco it should be because you like the way it looks. I dont feel it contributes much root fert and if thats your reason for getting it, get yourself some root tabs. Rootmedic on here sells them.

If you are looking for a nutrient rich substrate spring for Aquasoil.

As already suggested you can add root tabs for your root feeders instead of investing in substrate. That will save you some money that could go toward a pressurized CO2 rig. Plants need NPK and micro ferts and carbon (CO2) to do well. If you are not dosing the water column or the substrate and not providing CO2 that could be the reason your plants are struggling. A combo of ferts plus CO2 will give you better results than you will get by replacing your substrate.

how many bags of aquasoil would i need for a 3inch substrate in a 29G?

Azoo Plant Grower Bed is another option. Just as good as Aquasoil without the ammonia spike.

Adding root tablets is so cheap and easy the sensible thing is to try that first.

you can try Osmocote Plus which is similar to rootmedic, they both will help your plants. i know rootmedic is safe for sure, osmocote could cause ammonia problem if you added way to much. this is another cheap method to try.

You will solve your struggling root feeding plant problem by feeding the roots as mentioned. Macro nutrient root tabs. Or Osmocote fertsicles, or in gel caps. And, of course, you did not mention, CO2. In combination with the root ferts and your swords and crypts will explode w/o having to replace the substrate.

+6 for root tabs. Head over to rootmedic.com and order from Justin. I use pool filter sand with root tabs in my 36g tank and all my swords and crypts grow extremely well with very low Co2 and very light dosing of other ferts....