Really, it's only 2 tiers. You start with drained, so no grind involved. You level up to the rare set, which will be quite doable solo. Then you will grind up to legendary. Which will be a little more of a grind, especially for solos, probably needing lfg at times. What you are considering the 3rd tier, is to get the set masterworked. Which will be a headache, playing solo. But I'm sure many will be using lfg, so it will be doable, just above average difficulty, for a, "mostly," solo player. But you won't actually need to masterwork the sets, to have the gold trimmed stuff, or to utilize the armor glows. At least if it works like the 2018 Solstice event. And pretty sure, you wont need the set to be masterworked, in order to unlock the 2.0 Armor for September. So really it's 2 tiers + masterworking.

You get an entire month to play it. And you have your weekends. The last tier is going to be the most tedious

With a full time job it took me 14 days solo for solstice the first time. That’s all 3 legendary sets. That was 6-9 hours a day though. So half that would’ve taken the full month, probably.

Should be more than enough if you focus on it.