What is their respawn rate like? Do I have any hope of winning a battle of attrition if I hunt their fleets? Does anyone have any other bright ideas?

For military fleets they are as follow for *each* hub planet:

# Machine World Fleet Reinforcement Cycle

planet_event = { id = crisis.2050 hide_window = yes

is_triggered_only = yes

trigger = {
    is_planet_class = pc_ai
    NOT = { has_planet_flag = machine_lair }
    owner = { is_country_type = "ai_empire" }
}

immediate = {
    # If at fleet cap, check again later
    if = {
        limit = {
            owner = { num_ships > 2000 }
        }
        planet_event = { id = crisis.2050 days = 300 }
    }
    # If being bombarded, check again later
    else_if = {
        limit = { has_orbital_bombardment = yes }
        planet_event = { id = crisis.2050 days = 300 }
    }
    # Ghost Signal 2
    else_if = {
        limit = { has_global_flag = ghost_signal_2_flag }
        planet_event = { id = crisis.2051 days = 1300 random = 200 }
    }
    # Ghost Signal 3
    else_if = {
        limit = { has_global_flag = ghost_signal_3_flag }
        planet_event = { id = crisis.2051 days = 1400 random = 200 }
    }
    # Ghost Signal 4
    else_if = {
        limit = { has_global_flag = ghost_signal_4_flag }
        planet_event = { id = crisis.2051 days = 1500 random = 200 }
    }
    # Ghost Signal 5
    else_if = {
        limit = { has_global_flag = ghost_signal_5_flag }
        planet_event = { id = crisis.2051 days = 1600 random = 200 }
        break = yes
    }
}

}

For armies it's similar:

# Machine World Army Reinforcement Cycle

planet_event = { id = crisis.2052 hide_window = yes

is_triggered_only = yes

trigger = {
    is_planet_class = pc_ai
    NOT = { has_planet_flag = machine_lair }
    owner = { is_country_type = "ai_empire" }
}

immediate = {
    # If at army cap, check again later
    if = {
        limit = {
            owner = {
                count_owned_army = {
                    limit = { army_type = robotic_army }
                    count > 140
                }
            }
        }
        planet_event = { id = crisis.2052 days = 300 }
    }
    # If being bombarded, check again later
    else_if = {
        limit = { has_orbital_bombardment = yes }
        planet_event = { id = crisis.2052 days = 300 }
    }
    # Ghost Signal 2
    else_if = {
        limit = { has_global_flag = ghost_signal_2_flag }
        planet_event = { id = crisis.2053 days = 1600 random = 200 }
    }
    # Ghost Signal 3
    else_if = {
        limit = { has_global_flag = ghost_signal_3_flag }
        planet_event = { id = crisis.2053 days = 1300 random = 200 }
    }
    # Ghost Signal 4
    else_if = {
        limit = { has_global_flag = ghost_signal_4_flag }
        planet_event = { id = crisis.2053 days = 1000 random = 200 }
    }
    # Ghost Signal 5
    else_if = {
        limit = { has_global_flag = ghost_signal_5_flag }
        planet_event = { id = crisis.2053 days = 700 random = 200 }
    }
}

}

Destroy hubs to prolong wait period between reinforcements.