I'm running in to some major issues in regards to the design choices of a game I'm currently working on.

Just to begin, let me define some basic terms I'll use so I don't have to type out long winded descriptions of everything on a regular basis.

SM - Strategic Map - A large scale map of the battle field. Think something akin to 2-3 miles across in each direction for scale.
SU - Strategic Unit - Units found on the strategic map. These are made up of smaller, individual battle units. For theme, think of a SU as say, a large of spearmen or a group of swordsmen. (100-400ish men in total)

BM - Battle Map - A zoomed in version of the strategic map, focused on a small area. Approximately a football field across in size. Much much smaller than the strategic one. The aim is that every tile on the SM represents an individual battle map. Battle maps are contiguous in nature, so if put together, would form coherent looking terrain that represents the SM.

BU - Battle Unit - A small group of troops founds on Battle Maps. Represents a cohesive group of guys led by a sub-leader. Represents 20-40ish men or so.

My problem is this...
How exactly do I handle the interaction between SU's on the SM and converting them to BU's on the BM, without making the turn based structure of each overwhelming for the player(s)? The issue is that, let's say the player can lead up to 6 SU, and each SU is made up of 6 BU's. I don't want to make the entire battle one gigantic map, as it would be daunting to move so many units (36ish) around at once on such a huge grid. (300+ by 300+).

Thought Process
I felt it would be much simpler to shrink the unit size down into smaller groups, then have each of them initiate combat with one another. Any time two SU's collide, they have the option of initiating combat, which 'removes' them from the SM and creates a BM (and the appropriate BU's that they represent) in its place. Every turn the player moves all of their units, regardless as to how many maps there are. Here is where things also get tricky. What about the case where you have two SU's to their one? How is this an 'advantage' besides you being able to wear them down first before striking with the second unit? Surely they don't have to wait around outside the map right?

Battle Maps are designed to expand and grow with the introduction of additional SU's attempting to enter. Hopefully this poorly drawn ASCII grid explains what I mean.

Fig A.
+----+----+
| BM | |
| +------|
| BM | SU |
+-----+----+

Fig B.
+----+----+
| BM | |
| +------|
| BM BM |
+-----+----+

Two units are alright fighting in the tiles represented by BM, while SU wants to join the fight. (Fig A) As long as they are not engaged in combat already (Which they aren't seeing as they're still on the Strategic Map in the first place) they can join the battle, expanding the area of the battle map to include the tile they are currently on (Which represents many tiles on a BM, so this would be expanding the current battle map size by 50% in this example) and turning that tile in to a battle area on the strategic map. (Fig B) Other units could then also pile in to that specific battle if the opportunity arises.

This creates two problems for me.

1. Should this 'feature' work this way? Is it a pain in the rear and should be dropped? Theretically, a player could engineer a situation where even if they have a 'line' of strategic units against another line (Two horizontal lines falling on top of one another) they could all enter the same battle as opposed to fighting only the SU in front of them. This doesn't avoid the "Oh man I have 36 units on a 500x500 grid to deal with." situation that I wanted to avoid in the first place. Locking units in to their map until they retreat, die, or route their enemy though creates situations where you have troops waiting around doing nothing because all of the enemies are tied up already. You can flank them on a micro scale, (Individual BU's) but not on a macro scale (SU's).

2. Let's say I keep the feature and the following situation occurs. You have three units in a horizontal line, while the enemy has two, also in a line, but the middle unit is missing. They move and engage the units on your sides, creating two separate battle maps simultaneously. During your turn, you fight both small battles, but then what happens with your third unit? Is it unrealistic to say they can only participate in one of the smaller battles, even though they are technically wedged between the two? What happens to the smaller maps? Are they merged (and so now it's one huge 3 v 2 brawl with strange map boundaries?) or does it remain a a separate 2v1 and 1v1 with touching boundaries? I'm not against the latter and almost prefer it except in the case where, you're killing the last of their units with one SU worth of troops, and want to help your buddies in the other battle with your remaining guys. (Your spearmen are chasing down their axemen and you want to send your swordsmen who have nothing to do over) I guess I could use the same rules for retreating, but that feels a little clunky.

In all, I have this theme I'm not willing to part with, but I'm not sure how to make it work. Any suggestions or thoughts on how this might play out / what you think of it at a first glance?

A majority of the units will be melee combat range, and an even greater number of them, although ranged, won't have a firing range greater than a single Battle Map. (IE. Archers engage a unit of swordsmen, the swords still have to make that initial approach to actually attack the weak and frail archers.)

My vision / advised use case for archers would be to join an existing battle where enemy units on the battle map are already tied up so they can fire away with imputiny.

As far as retreating goes, yes they can. I hadn't hashed out the exact rules for it yet, but something akin to if the entire remaining SU group of BU's are at the edge of the map and there is space on the S-Map for them to exit in to, then after a turn(s) of no combat, they leave the BM and enter the SM. I suppose the ability to 'retreat', even if it is in to another map would solve this issue. However, if two battle maps are adjacent to one another (No space between them on the strategy map), then should they be one huge battle map, or should there be this seemingly 'invisible' barrier that separates them via strict retreating rules. (So they can exit, then re-enter somewhere else on the other map.)

The way it works in advance wars (and probably many other SRPGs I haven't played) is that when a unit engages another unit on the overall map, they are the only side that is dealing damage, and then the enemy can do the same in their turn (which would sort of make it a battle) or flee.

The other way I can think of is that you can have the game logic in BM and BU, but relay the information to the player in SM and SU when appropriate.

Usually, the two maps are on completely different timescales. For example, on the strategic layer, troops marching takes days, while a battle is usually resolved in minutes/hours. And games just force all battles to be resolved before moving on to the strategic layer.