I haven't had a chance to verify but I'm borrowing some of your logic for my Nauvis ecosystem mod (it's called Terra Nauvis! if you want to look) but

148     {
149         type = "bool-setting",
150         name = "biter-repop-destroy-buildings",
151         setting_type = "startup",
152         default_value = true, -- matches current behavior
153         order = "e3",
154     },

But in biter-spitter-migration.lua the destructive update_effects are assigned unconditionally:

265         -- Damages buildings/belts/rails it passes through; excludes
266         -- trees/rocks since they don't use these collision layers.
267         update_effects = {
268             {
269                 distance_cooldown = 0,
270                 effect = {
271                     {
272                         type = "nested-result",
273                         action = {
274                             type = "area",
275                             radius = 3,
276                             force = "enemy",
277                             collision_mask = {
278                                 layers = {
279                                     is_lower_object = true, is_object = true, player = true,
280                                     rail = true, train = true, transport_belt = true,
281                                 },
282                             },
283                             action_delivery = {
284                                 type = "instant",
285                                 target_effects = {
286                                     { type = "damage", damage = { amount = 500, type = "impact" } },
287                                 },
288                             },
289                         },
290                     },
291                 },
292             },
293         },

Just thought I would let you know while I was looking through it!

Thanks for letting me know! (But also, what does this mean?)

Intended effect is I'm trying to have the units damage buildings, but not anything natural

Ooooh, as in I have a leftover setting that doesn't do anything

it means you put the option there but it never checks whether the box is checked or not- it will always be on (they will destroy buildings)

hey have you tried flying around over water? I'm having trouble with biters wanting to just walk right over the ocean if their territory overlaps the water

biters walking over water is intended behavior. it can help the migration paths be more connected between continents. i believe my code limits their spawning to elevations above -5. so they won't cross water often, but in my testing this was enough to drastically improve migration path connectivity. I just imagine it as many animals irl cross rivers and such in their own migration paths. If I was to improve the mod I'd probably go more in the route of adding a swimming animation to biters.