I've had an EVOC soft case for eons. It's served me well.
The last bike i transported, i put in a bike box i got from a shop that was throwing it away. it might be the best option, especially if you already have wheel bags (protection). It would cost you between nothing and pretty much nothing, which is up to infinitely less than a hard case. Bike bags are insultingly expensive. I remember paying a lot for my EVOC bag, and since then it's just become a farce. A problem is long term storage for the 99% of the time when you dont use it.
There's a ghetto aspect to it I enjoy. And you're recycling. And when you're done you can compost it. How many bike boxes are compostable?
You can splurge on packing materials given your spend is so far zero to protect stuff. The box doesnt have wheels though, but i'm sure you can buy something on aliexpress and tape it on. Duct tape is always a friend.
Because it's a hard box, you can ship it instead of traveling with it, potentially.
After you've gone through the motions a few times, you realize there aren't many things that can go wrong if you protect the bike in the right places and stuff the box so that there's no rattling. If you're a civilized human you probably wax your chain which also means your bike is clean to start with.
I think i'd trust a reused bike box from a shop more than a soft case, because a soft case is just awkward and subject to odd forces, but a bike box is engineered to stay in its box format.
