From the sound of it, you are doing the right thing in providing adequate housing for your toads. There would be a couple of points to optimise breeding.
Increasing the ratio of male to female toads. It is recommended to have 2-3 male toads per female toad, in the wild this ratio is larger than this and assists in the mating process. As males are smaller, it's easier to keep an extra male.
Rather than trying to raise the eggs within the vivarium with the adults I would recommend removing the eggs.
It would appear that removing fertilised eggs from the tank and raising the babies separately from the adults is a more viable way of breeding in captivity. Both the egg to tadpole stage and then the tadpole to land stage of development require some specialised and separated housing. Given the complex set up for the adults, the housing for the babies should not be too difficult for you.
You'd require a small aquarium for the eggs and tadpoles and plastic ice cream type containers for the land dwelling babies.
I have quoted from two sources. The second link also has email contact on the website.
Typically, eggs are laid
on submerged plants near water's edge. Eggs hatch after a period of 3
to 10 days. In the first week following hatching, the tiny larvae
absorb their yolk sacs. ...
After fertilization, the jelly layers of the eggs absorb water and swell. The eggs remain in this "jelly" state until hatching. (1)
If mating is successful females will deposit their 40 - 110 eggs either individually or in small clumps of 4 - 25 eggs very close to the water surface where the warmth of the sun (spotlight) can aid embryo development. It is advisable to remove the eggs to a separate 90cm aquarium (per 100 eggs) containing 20 - 30cm of cool (60 - 65°F.), well-oxygenated and most importantly fresh water. Within 5 - 8 days the 6mm black tadpoles hatch out to spend the first week of their life clinging inanimately to the glass sides, plants or rocks while absorbing their yolk sac. ...
For the first few weeks of their land-based life Bombina toadlets can prove quite difficult to raise. They are best removed to plastic ice-cream tubs with a secure but well-ventilated lid containing a base of soaking kitchen tissue paper. (2)
(1) Bombina orientalis
By Todd Szcodronski
Animal Diversity Web
University of Michigan
Museum of Zoology
(2) The Fire Bellied Toad
(Bombina Species) FAQ
by Marc Staniszewski