Sharing a mtd hack to allow firmware to auto detect and utilise the increase flash size if a hardware flash mod is done like the behavior of the old ar71xx on the ath79
It promises not to touch original routers but will help modified routers utilize the extra space
Feel free to upstream it 
It detects only partition in the DTS with the name "firmware" can probably be adapted to detect other names in the DTS.
--- /dev/null
+++ b/./target/linux/ath79/patches-4.19/950-linux-mtd-hack.patch
@@ -0,0 +1,43 @@
+--- a/drivers/mtd/ofpart.c
++++ b/drivers/mtd/ofpart.c
+@@ -36,7 +36,8 @@ static int parse_fixed_partitions(struct
+ struct device_node *pp;
+ int nr_parts, i, ret = 0;
+ bool dedicated = true;
+-
++ int fw_idx,last_part_idx,count; // track which index is firmware partition on and the index of the last partition
++ uint64_t total_size,diff; // use to calculate total size which is offset of last partition + size and store the difference
+
+ /* Pull of_node from the master device node */
+ mtd_node = mtd_get_of_node(master);
+@@ -114,18 +115,28 @@ static int parse_fixed_partitions(struct
+ if (!partname)
+ partname = of_get_property(pp, "name", &len);
+ parts[i].name = partname;
+-
++ if (!strcmp(partname, "firmware"))fw_idx=i; //get the index of firmware
+ if (of_get_property(pp, "read-only", &len))
+ parts[i].mask_flags |= MTD_WRITEABLE;
+
+ if (of_get_property(pp, "lock", &len))
+ parts[i].mask_flags |= MTD_POWERUP_LOCK;
+-
++ last_part_idx=i; //get the index of the last partition
+ i++;
+ }
+
+ if (!nr_parts)
+ goto ofpart_none;
++
++ total_size=parts[last_part_idx].offset+parts[last_part_idx].size; //calculate total size = last partition offset + size
++ if(master->size > total_size) //if flash is modified then adjust
++ {
++ diff = master->size - total_size; //get the difference
++ parts[fw_idx].size = parts[fw_idx].size + diff; //increase the size of firmware
++ for (count=fw_idx+1;count<=last_part_idx;count++){
++ parts[count].offset=parts[count].offset+diff; //increase all the offsets of all partitions after firmware
++ }
++ }
+
+ *pparts = parts;
+ return nr_parts;