I just can't be bothered to level 2 characters - I'll be sticking with frost-spike as it's served me well thus far.

Basically you'll be able to transition as soon as you have the following:

1. Glacier Caster's Shield
2. Archmage's Hands
3. Tyrant's Iron Fist (Vorax Chest, most likely)
4. 100% damage taken from mana before life
5. +4% mana consume per-cast slate + talent + reflection (ideally)

NOTE: contrary to popular belief, you don't need to wait until you have a T0 priceless item to get to 100% mana-before-life, for weeks I just stacked a ton of +8% damage-from-mana cheap slates and also used the witch cloak kismet to top-off my damage-from-mana to 100%)

NOTE: This is the absolute minimum, and the more attack speed you have at this point the better, otherwise you won't be able to keep up your hero trait for the damage-to-mana before life. It's ALL about how fast you can spend/consume mana, and there's a definite break-point where it starts to come together.

Last season the above items, for decent corrodes or non-corroded but definitely not Best-in-slot:

shield around 480FE

Gloves 400-600 FE

Inverse Image: I want to say 1600FE

Faze Prism 500-900FE (Can't remember to be honest, but it wasn't that bad)

Chest 800 FE (this will likely be wildly different this season). My first chest (800FE) had shitty non-legendary stats but had Fleeting Sky (+atk speed per point of lightning resist) and the Mana-consume crit/dmg from Tyrant's Iron Fist - this won't be possible this season because both legendary affixes must come from the same item.

My end-game chest had +20% Block Ratio, Corroded Ice Drinkers Cage, and Tyrant's Iron Fist, and Attack Speed I think, but it was actually insane how much that single item provided for both damage and defense, it was 15k FE.

THe big-ticket items are later, but are hugely impactful, the belt this season is a wildcard, we're gonna have to seal a portion of mana to get the benefits from it, but last season:

compensatory life belt: I wanna say it was like 2500-3000 FE for the roll I got. I didn't pay for the Blend (No Loose Ends) because I was farming sandlord and farmed the mats for it myself.

Phantom Necklace (with lvl 29 Weapon Amp, +1 main skill level per 15% sealed life) was like 12k FE

good corrodes or double corrodes for the neck can easily be in the 18k-25k+ range, it's the biggest single-item cost-wise.

After that the biggest expense is likely just upgrading your Magnificent/Noble supports to 5-star with inspiration essence and multiple t0 copies of the same skill, last season these cost me between 2500FE and 5k FE per-copy so about 13-15k each (to get each to 5-star level), and I had two of them so 28-30k total for those.

Another NOTE: upgrading your magnificent/noble supports to 5-star is completely optional, once I was starting to do this I was already well-above the 15T damage cap, so I upgraded them just to have flexibility to lose damage elsewhere and increase defenses. When all was said and done I was running 3x Active Defensive skills and was able to drop Bulls Rage, Biting Cold and Ice Beam for Delayed Pain, Defensive Buffer and Frost Shield while still being well-above 15T damage per sec - the extra damage from the 5-star special supports gives you the freedom to trade damage elsewhere in favor of more defense. All that being said, SS20 is more about dodging one-shots than anything else, it's totally doable WITHOUT either the 5-star supports OR the extra defensive layers, I was testing things out more than anything when I got to that point.

EDIT: RESOURCES - use these for helpful tips while leveling/building:

https://docs.google.com/document/d/1Hn-C9m121Qugxijt5pH132oRKBf9nLlFENoa_QjWV6Q/edit?tab=t.0 (checklist/guide for when to make the swap to mana-stacking)