Linux dynamic rate limiting/throttling

The config you have now is only going to make sure you only have a single simultaneous delivery to each destination. It's not going to actually throttle anything. I think adding this line to main.cf may accomplish what you want:

slow_destination_rate_delay=2s

That will tell the queue manager to insert the specified delay between each individual delivery to the same destination. Since you haven't modified default_destination_recipient_limit from its default, it should apply the delay to each recipient domain. So this should mandate a 2 second delay between each delivery to the domain listed in the transport map.

When I've had to do this in the past, I didn't go with the alternate transport mechanism. Since I was only concerned with delivery speed to each unique domain, I just added these lines to main.cf:

smtp_destination_concurrency_limit = 1
smtp_destination_rate_delay = 1s

So if there is mail being injected for 30 different domains, it may try to deliver a message to all 30 of them at once, but never more than one at a time to example.com and never faster than 1 per second to example.com.

Unless I am misunderstanding your goals, you cannot really restrict the rate at which packets come in to your network from your ISP unless you coordinate some form of QoS with your ISP to shape traffic at their routers. This typically requires a business-class connection and money. You are seeing packet drops because that is the only way to shape traffic - you drop it on the floor. So your TC rule is working as designed, but you cannot control what level of traffic gets sent to your WAN interface, as senders try to send it as fast as possible.

Now, you can sort of shape incoming TCP traffic by strategically dropping packets or messing with ACK packets to make senders think your network is more congested that it actually is. But that doesn't really help things for non-TCP traffic coming into your network.