My advice is to decide on the type of cleric you want first, then use the mythic options to make the character "better" at what you focused on. There are quite a few different ways you can make an "effective" cleric.
Take a look at the cleric on Archives of Nethys, especially the archetypes, domains, and variant channeling.
In a mythic game I would recommend an Oracle over a cleric. Inspired spell allows an oracle to cast any spell on their list. This lessens the impact of only knowing a limited number of spells. A Oracle has a lot more class features than a cleric due to revelations, and the ability to gain more by taking extra revelations. Mythic Domain boosts all revelations by 4 levels.
Oracle’s have a theme backed into the class from their mystery and curse.
If you do want to stick with cleric an Aasimar cleric with the sun and glory domains focusing on channeling can be very effective. If the campaign has a lot of undead and evil outsiders your channel energy will do a lot of damage. Between the sun domain and the Aasimar favored class bonus you end up doing an extra 1.5 points of damage per level. I have a player who is running one in a undead focused mythic campaign I am running and he does quite well.
Personally I would probably go with an oracle of heavens and spam out mythic color spray. The heavens mystery has a lot of decent combat revelations and spells. Take the blackened curse to add some fire spells to you list of spells known. As I mentioned before Inspired spell will allow you access to any cleric spell so you can still handle condition removal without having to waste spells known or depend on items.
I would not advise trading away half your feats in a mythic game. Most mythic feats require you to have the base feat first so that is going to limit your options. You are going to want to pick up the Hierophant path ability Bleed Holy Power. This allows you to channel energy as an immediate action when damaged by an undead. When you do so you can still channel energy normally, but if you do so you will be using your channel energy uses very quickly. Extra Channel becomes an almost must have feat and you may even want to take it multiple times.
You will also probably want alignment channel and the mythic alignment channel. Mythic improved channel is also a feat you should take. It causes any non-mythic foe to roll their saving throw twice and take the lower result. Mythic Extra Channel may also be useful. It allows you to use two mythic points to channel energy when you have no uses left. You will also want to have feats to boost your spell casting. Mythic Spell Penetration and Mythic spell Focus are very good to have. Both of those boosts both the normal and greater version of the normal feat while only requiring a single mythic feat.
Even though you are focusing on channel energy you still need option for when that does not work. Using all your resources for channel energy is going to weaken the character. It also means you will probably be using it too much so may run out and not have it available when you need it most. By giving yourself other options you can really shine when the situation to use channel energy does come up.