Is there a way to mark different networks with different metrics in the same route-map for the same neighbor?
Like if I want to mark 10.10.10.0/24 with metric of 900 but want to mark 126.96.36.199/24 with metric of 100, can I do this in the same route map?
you are right with route-maps depending on the way we use them we need to deal with the implicit deny any logic at the end of route-map.
The implicit deny any of route-maps have no effect only when using a route-map for PBR, because it means standard destination based routing.
In this context, an explicit last route-map block with permit may be needed to avoid unwanted filtering.
Final note: BGP MED is intended to be used in eBGP with directly connected AS and not in iBGP.
I remember your network scenario it was difficult to demonstrate that iBGP multipath was working.
However, I remember your firewalls are Palo Alto and may not support iBGP multipath.
If you want to influence only the firewalls choices you should look for the equivalent of Cisco weight.
If Palo Alto follows the Juniper way you should look to play with route preference ( equivalent to Cisco AD but a 32 bit value).
If you change the MED on core switches you are going to break the iBGP multipath also on other devices that could use it.
If it is acceptable for you to build a load sharing solution based on IP subnets, because in all cases you need to go via the firewalls, the suggested route-map can work also in iBGP. Pay attention to the way you close the route-map (if you need to pass some routes without changes in attributes).
Hope to help
If you want your upstream device to prefer a prefix from the active HSRP, I would do an EEM triggered on HSRP that will change a route-map modifying outbount as-path (prepend). If HSRP active no prepend and if standby then prepend. When it will be learned on your upstream device, it will prefer the one no prepended or with the shortest as-path
Don't have any doc right now to refer to but if you give your config sample, I can build something for you.
Give me a little bit of time to do so as I'm working on several projects at the same time.