HRE main here. I dunno if devs are reading this forum but here it goes. I play HRE like 90% of the time and mainly play 1v1. So Im looking at balance from that viewpoint and my suggestions are focused on that balance, maybe it will be good for large team games as well.
First of all lets look at how the Civ is designed. HRE was designed to be a civ that focuses on infantry and the economy with strong landmarks. They are also designed to have a higher priority at getting relics than most civs.
How the civ is played in top and middle elo is a lot different from designed. They are mainly played with Aachen rush then Regnitz and relics then Swabia boom. There is no other way to play it competitively. HRE in high level usually does knights spam instead of m@a.
Here is my suggestion to make HRE more well rounded instead of just fast castle regnitz and boom. This is done by improving HRE feudal aggression play so that you do not need to FC 99% of the time and then to balance Landmarks so that one landmark isnt clearly stronger over the other. So here are my suggestions to achieve such.
First marching drills should increase movement by 15% instead of 10%. Pros use knights over M@a since the later move so slow over the former. Knights being 30% faster than even the marching drill M@a. If you increase marching drill to 15% that means that knights are now just 26% faster than M@a so you gain added incentive to choose them over knights. This would also mean that HRE M@a are now a bit faster than their Delhi counterparts over long duration of time, 15% faster than vanilla compared to 13% for Delhi (100% x [10/75] ) while Delhi is still faster in bursts which is how it was meant to be.
Second move two-handed weapon to Feudal. The tech doesnt fit castle age and is a lot worse than the Delhi counter part that gives +3 and affects knights as well. Moving this to feudal would give HRE a feudal power spike and gives incentive to go Feudal aggression and more flexibility than just going FC Regnitz 99% of the time.
Third give them their buffed up version of Elite Army Tactics. Giving something like 25% bonus hp and damage than 20%. Mongols already have their version that gives 30%, this would push HRE more into infantry and does marginal bonus to make their knight spam stronger.
Fourth, landmark rebalance. This is mainly to address castle and imperial age landmark balance since Meinwerk Palace would be a lot more viable now if suggestions 1 and 2 are implemented.
Regnitz Cathedral should give a 100% bonus than 200% but have the same storage space as a normal monastery. This means that at 3 relics you gain 600 gold instead of 900 while at 4 relics you gain 800 instead of 900. Currently 200% was just too powerful and further the 3 relic limit made the HRE have little added incentive to get the 4th and 5th compared to other factions.
Burgrave Palace: trains batches of 5 but at the price of 4. Basically gives an additional 20% discount on top of functioning as 5 free barracks. This would give HRE added power with Castle aggression but does nothing for their already powerful Regnitz relic strategy. This also makes the choice of a castle age landmark not be a no brainer choice.
Swabia: Move the 20% discount to Elzbach. Remove the villager discount. This way it still functions as 4 town centers but still make raiding imperial age HRE useful due to the need to spend more food to replenish villagers. Cause currently raiding HRE was just futile since imperial HRE can just replace villages very fast and very cheaply. Removing the discount makes them pay more food for the economic damage.
Elzbach: Get the 20% discount from Swabia (maybe turn this to 25%), this way players who need to go imperial really fast would choose this wonder aside from the defense bonus.