Merging isolines into polygons to produce isochrone map in QGIS

There is a set of isolines produced with v.net.iso.

It shows distances in meters [400, 800, 1200, and everything further] from a certain point.

My desired output is an Isochrone map. However, I do not know yet how to merge/aggregate isolines into polygons, where each isoline located exactly in a proper polygon with no overlaps etc. Better to say I want to draw an outline from the farthest points of each isoline class. What is the best approach?

I have tried:

1. extracting nodes from lines Vector > Geometry Tools > Extract nodes, filtering with a certain isoline class, and then Vector > Geometry Tools > Concave Hull.
2. deploying the QGIS plugin, namely QGIS-ConcaveHull-Plugin.

Both outputs are represented on the image below, numbered appropriately. But there is a problem, nodes from the higher-class isoline group overlap the lower-class isoline group. How can I handle this issue?

I have seen these articles but nothing spectacular:

• Drawing outline of point cloud using QGIS?
• Creating drive time polygons using Open Source tools?
• Computing Walking and Cycling Isochrone maps using QGIS & pgRouting?
• How to create contour lines from point data using QGIS?
• Create polygon layer from polyline layer