I’ve done these a few days ago with a Canon 77D and Canon EF-S 10-18mm using multiple exposures (like at least 5).
I’m so sorry to say to you that you won’t be able to do this in any opensource software, at least not in any reasonable amount of time spent on editing.
1st of all, you won’t be able to merge HDRs easily, and in my experience I get green fringing and artefacts around clipped areas from any image.
2ndly, perspective correction often does a sub par job correcting and there is no option to manually add vectors in Darktable so that’s a hit or miss. (good luck doing it with sliders)
3ly There’s no raster brush masking tool with edge detection, so again, I’m sorry to say but you can potentially spend hours tweaking the masks in Darktable that could easily be painted with a brush that has edge detection.
That said, Darktable is better than Lightroom in everything else, just not what you currently need. I’m using it more and more but I really hope that DT one day gets a good way of merging images into HDR dng-s, a raster paint brush masking tool with edge detection and a marginally better perspective correction module (where you could draw the lines yourself).
I’ve tried to do these in Darktable but as I was nearing the deadline I just had to do it in Lightroom and be done with it:
