Arena score?

And you are currently going between tiers 19 and 20? If that's so, I think that there's no rush to do any immediate changes. You have already invested in Tiki quite a lot, so if you like her, it would be good to finish merging her after you get those last few copies.

As for the rest, I honestly think that you should find heroes, that you actually like, although I noted that you don't have any absolute favorites. The reason for this is that if you don't like the hero that you are going to be merging up, you will lose all the excitement and drive from getting a next merge copy or getting enough feathers for another merge. The whole journey will then feel like a boring chore.

The arena really isn't anything special and if you are indeed between tiers 19 and 20, there is no need to rush any big changes, because the rewards that you are already getting are good enough for normal progress through the game.

EDIT: I can give my own experience as an example. When dragons got their refinement options and Nowi became one of the most powerful heroes, I felt like I had to merge her up despite hating absolutely everything about her. Her art is awful, and her personality and voice acting are annoying. In the end I was kind of bummed down that I had to dump more and more feathers to finish her and the whole time that I was completing my merge goal, was probably the most boring time I had in FEH, because it felt just like a routine and not something that I would be looking forward to.
After that I basically said "fuck arena" and I just merged whoever I actually liked.

Your question is a disaster to try and read.

Arena score focuses on these following things, and ONLY them.
1. BST
2. SP cost per unit
3. Merges
4. Seasonal Bonus
5. Rarity Level

And i'll break it down for you into bite sized chunks.
BST is what each unit has, BST is ranked in groups of 5, rounding down to the nearest 5th, for example, 158 = 155, 162 = 160. Each grouping increases your arena score by 2, give or take an average grouping between all four units.

SP cost is the total SP spent on the equipped skills each unit has. This score ignores inherited bonus skill cost and only applies the base normal cost. Usually, the higher the total of all your skills, the higher the unit scores, but not always.

Merges are the single biggest boost to arena score, each second merge, +2, +4, +6, +8, and +10 will give an additional 2 points to that units arena score, and to the total arena score value your team will have.

Seasonal Bonus doubles the bonus units score potential, plain and simple.

Rarity level is a bit different, 5 stars are the best, however in terms of scoring 4+10 is equal 5+5, so consider making a bunch of 4+10's of characters you have no interest inn actually using, that way if therye ever a bonus unit and you have no others, you wont have a nerfed score as badly as if you just used an askr unit.

This is a tool used to figure out your teams scoring potential. Use it, learn the general in's and out's of scoring, and remember that you NEED armors to stay in tier 21, you NEED then, or you NEED other units that can reach a 170 bst score in arena. Its REQUIRED, YOU CANNOT SCORE HIGH ENOUGH WITHOUT ARMOR-TIER BST TO STAY IN TIER 21.
https://www.arcticsilverfox.com/score_calc/