here's a plan
1. you give each client equal minimum share of the bandwidth
2. you let clients have equal share of un-used bandwidth (the share that other clients aren't using).
3. within their share, you prioritise latency sensitive traffic
That way my video conferencing can't oust someone else's emails.
"tc" with "htb" on linux can do this. I don't know if it can cope with the number of customers that ISP's have but I was working on site shaping solutions using this a few years ago and it works very well.
A standard configuration "mended" networks when 10x the bandwidth couldn't.
Doubling the bandwidth might halve the time a large email contends with voip - but it doesn't remove the contention.
Shaping lets voip packets jump to the front of the queue but doesn't actually increase the total time to send the email.