i wanted to create a timelapse with the file-names burned into the image..
should be a quick ffmpeg commandline – i thought..

lots of hours later..
you can now find the result in the battery run-time post.

Workflow is as following:
using blender with a special python script to burn the filenames onto the output.

# based on https://blender.stackexchange.com/a/8087/16634

import bpy

#debug helper
print("all vse strips:", end="\n  ")
print(*bpy.context.scene.sequence_editor.sequences, sep="\n  ")

# output VSE Text strip:
text_overlay = bpy.context.scene.sequence_editor.sequences["Text"]
#x = bpy.context.scene.sequence_editor.sequences["day2"]

def get_combined_speed_factor(strip):
    # this does not handle all possible configuration options!
    # only 
    # - Stretch to input strip length
    # - Speed factor 
    # - Multiply Speed
    # are handled.
    #print("strip", strip)
    combined_speed_factor = 1.0
    #print(" use_default_fade", strip.use_default_fade)
    #print(" speed_factor", strip.speed_factor)
    #print(" multiply_speed", strip.multiply_speed)
    if strip.use_default_fade:
        # update / overwrite not used speed_factor value.
        orig = strip.input_1
        strip.speed_factor = orig.frame_duration / orig.frame_final_duration 
    # calculate
    combined_speed_factor = strip.speed_factor
    combined_speed_factor *= strip.multiply_speed
    return combined_speed_factor

def get_speed_factor_for_strip(scene, strip):
    strips_all = scene.sequence_editor.sequences
    strips_speed = (i for i in strips_all if i.type == 'SPEED')
    effects_for_strip = (i for i in strips_speed if i.input_1 == strip)
    combined_speed_factor = 1.0
    # this should calculate the overall speed for this input strip
    for effect_strip in effects_for_strip:
        combined_speed_factor *= get_combined_speed_factor(effect_strip)
    return combined_speed_factor

def find_current_img_filename(scene):
    frame_current = scene.frame_current
    #print("scene.frame_current", scene.frame_current)
    #print("scene.frame_current_final", scene.frame_current_final)
    file_name = None
    # print("\n")
    strips_all_sorted = list(sorted(scene.sequence_editor.sequences, 
        key=lambda x: x.frame_final_start))
    strips_image = (i for i in strips_all_sorted if i.type == 'IMAGE')
    strips_speed = (i for i in strips_all_sorted if i.type == 'SPEED')
    for strip in strips_image:
        strip_speed_factor = get_speed_factor_for_strip(scene, strip)
        #print("strip", strip)
        #print("strip_speed_factor", strip_speed_factor)
        #print("  frame_final_start", strip.frame_final_start)
        #print("  frame_final_end", strip.frame_final_end)
        if strip.frame_final_start <= frame_current < strip.frame_final_end:
            #print("strip", strip)
            #print("  frame_current", frame_current)
            #print("  strip_speed_factor", strip_speed_factor)
            #print("  frame_final_start", strip.frame_final_start)
            #print("  frame_final_end", strip.frame_final_end)
            strip_frame = (
                (frame_current - strip.frame_final_start) 
                * strip_speed_factor
            ) + strip.frame_final_start
            #print(" → strip_frame", strip_frame)
            file_name = strip.strip_elem_from_frame(strip_frame).filename
    return file_name

def recalculate_text(scene):
    filename = find_current_img_filename(scene)
    #print("filename", filename)
    text_new = "-"
    if (filename):
        text_new = filename.replace(".png", "")
    text_overlay.text = text_new


this allows for files in multiple directories with complex filenames (like date_time) and additional tweaking of the speed and so on…
rendering this as png sequence.
than use a classic ffmpeg image-sequence to create the final video with this (2-pass) commandline:

$ ffmpeg -framerate 30 -start_number 0 -i %04d.png -c:v libvpx-vp9 -pass 1 -b:v 1000K -threads 8 -speed 4 -tile-columns 6 -frame-parallel 1 -an -f webm temp
$ ffmpeg -framerate 30 -start_number 0 -i %04d.png -c:v libvpx-vp9 -pass 2 -b:v 1000K -threads 8 -speed 1 -tile-columns 6 -frame-parallel 1 -auto-alt-ref 1 -lag-in-frames 25 -an -f webm out.webm

next step would be to change the script to really generate the runtime in minutes from the timestamps…