ffpmeg force overwrite (badly) detected input framerate - input

Question

I am running IP Webcam on Android which provides an mpjpeg video stream. I have to limit the capture frame rate to 5fps to save on battery.
However ffmpeg will still detect the input stream to be 25 fps, which causes it to be saved in the wrong speed causing timestamps and audio to be desynchronized.
Input #0, mpjpeg, from 'https://***:***#smarthome:8080/video':
Duration: N/A, bitrate: N/A
Stream #0:0: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 1280x720 [SAR 1:1 DAR 16:9], 25 tbr, 25 tbn, 25 tbc
Input #1, ogg, from 'https://***:***#smarthome:8080/audio.opus':
Duration: N/A, start: 0.006500, bitrate: N/A
Stream #1:0: Audio: opus, 48000 Hz, mono, fltp
Metadata:
ENCODER : Lavf58.12.100
[stream_segment,ssegment # 0x19b49a0] Opening '/mnt/nas/SecurityCamera/2020-06-20_14-26-04.mkv' for writing
Output #0, stream_segment,ssegment, to '/mnt/nas/SecurityCamera/%Y-%m-%d_%H-%M-%S.mkv':
Metadata:
encoder : Lavf58.20.100
Stream #0:0: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 25 tbr, 1k tbn, 25 tbc
Stream #0:1: Audio: opus, 48000 Hz, mono, fltp
Metadata:
ENCODER : Lavf58.12.100
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #1:0 -> #0:1 (copy)
Press [q] to stop, [?] for help
[ogg # 0x1753e00] Thread message queue blocking; consider raising the thread_queue_size option (current value: 8)
frame= 8 fps=7.9 q=-1.0 size=N/A time=00:00:00.49 bitrate=N/A speed=0.489x
As you can see it detects Input #0 to be 25 tbr, 25 tbn, 25 tbc which results in an expected 25 fps, however the fps (shown high right now but it slowly approaching 5fps) is way below 25 which causes the speed to be <1x.
I have tried to use -r 5 -i ... and -vsync 2 and different values for -enc_time_base none of which had any impact. From https://trac.ffmpeg.org/ticket/403 I've learned that -r only works on inputs with unknown fps. But my input doesn't have unknown fps, it has the wrong fps.
Is there any way to force overwrite the input fps so that I can get a proper speed of 1x and synchronized timestamps and audio?