Do you use the desktop or the iOS version of Affinity Designer?
There are many more options with eps files, but since I do not have the exact exports, it would be guesswork, which I try to avoid here.
I have seen too many eps files that were not written well in the past 30 years. So, I have to buy Affinity if you have more questions.
Please share your target with this workflow so that I can better help.
Thanks a lot; as Architectural Visualization has a wide range, this information helps to get a rough idea about your work. What I see on your page tends to be a completely new challenge each time. Not the standard walls, windows, floors, etc. So you certainly have to invent for each project a workflow even.
Besides classic modeling steps or sculpting:
I first suggest taking a plane and cutting it to your needs in any shape with a Knife or Spline-driven Knife; eventually, use Selection Capsules or manual selections to delete while adding with the Poly Pen.
As a result, you might end up with a horrible mesh. Then use the Remesh, eventually with a Spline a Poly flow guide, and adjust a few times until you get a clean mesh. After that use the Thicken Object to adjust to an single or variable (i.e., organic) thickness.
As you undoubtedly know, modeling is like chess playing; always many steps ahead to win.
If you have something to model, or looking for some ideas, please share a sketch, blueprint, or project file. I'm happy to look into it.
Your question was general, and with that, it is often complicated to find a one-size-fits-all solution, s we all have our ideas of where it would stop working.
If you have a specific case, then please share that as a sketch or as a file.
My typical MO is that I love seeing the Project file or a sketch.
I do not like to answer questions or reply from memory. Over the past 17 years, running the Q&A forum here has proven to be the best way. This thread shows that not having specific data is wasting your time. Sorry about this.
I love to find creative solutions if there is no default option.
One question, why it has to be a Spline? What is the target? The more details you share, the better I can explore this.
Please keep asking questions, as I believe that a problem comes back until it is solved. So, I try not to avoid anything here, and please never take my "moderation" (AKA keeping things organized) personally. New theme, new thread. Same question, old thread. Yes, yours was slightly different the second time; hard to find the right balance.
So far, I have answered Cinema 4D questions over 35,000 times in a Forum. The times I had to say there is no option (AFAIK) are very low. You can check even the past twelve years at Cineversity.com.
I'm sure that I do not know all, as I might find tomorrow a solution for something I was not aware of today. However, each question leads me to explore, and I'm under the impression that there is no failure, only gained experience. I assume I have some experience. 😉
So, I am happy to get your and other questions; they lead me to things I wouldn't have explored otherwise. I also try to understand the overall tendency and put this in discussions during our meetings into the mix. I try to be a voice for our members here, but please use the "Share Your Idea option; otherwise, I sound like someone who generalizes things without factual background. Thank you.
With that being said, I tried today several other options. But even if you use the path of the Sweep and cut that, the resulting shape with have straight edges in x, y, or z, per clone, staircase-like, not tilted as the spline asks for...
My comments were only meant to suggest where to focus next. I'm not critiquing. Since I write in a forum, I know we have various experience levels here. Hence, I point out certain things, perhaps obvious to most artists. In the editor view, all might look fine because the refresh happens more than once. During rendering or export, there is only one time to "walk through" all the items in the scene. What is not available will be replaced by the material that might have been from a single or a few frames before; if nothing is there, then there is no reply. This is my current conclusion with this setup I got. To be clear, the results visible in the editor view are very nice.
It sounds like a more significant production, whereby I'm not clear about anything besides this scene you shared. So, my suggestion might be off, but how about Taiao from Insydium? https://insydium.ltd/products/taiao/