Do not spend any money on Tower of Fantasy. I didn't spend any money, and I have multiple SSRs, basically everyone but Zero, King, and Nemesis, and that's with poor luck, and losing the 50-50 chance of getting a limited character on the limited-time banner.
Also, my assumption that Genshin Impact having a competitor would lower prices plus increase rewards prove wrong. It turns out Tower of Fantasy has issues with hackers. One guy tried to hack me in a PVP match, and I ended up somewhat winning it which I felt was pretty funny. I feel like Genshin will be like Destiny, it will be unkillable, and have highs and lows.
Also, Tower of Fantasy keeps nerfing their limited-time banner characters to the ground for no reason. They are afraid of power creep but don't want to follow in Genshin's footsteps of having enablers, useful lower-tier supports, etc.