I would recommend doing the following changes.
- Expect the characters to reach level 12.
- Give the characters very powerful magic items.
- Reduce the CR 17 big bad to a CR 15 big bad. If the big bad needs to be level 17 to have access to a spell, ability, or collection of spells, then give the big bad an item which replicates that spell. (For example, perhaps the power comes from the building they occupy, combined with some artefact, rather than their own skill) Alternatively, you can reduce the CR by giving damage vulnerability, or fewer HP.
This should allow them to level up over 2 or 3 sessions, and give enough fights between levels to feel the achievement. The extra magic items will allow the big bad to be a challenge but beatable and you won't have to adjust your planned content too much.
My advice is partially based on the fact that the published adventure themselves don't actually take characters to level 20.
Give them 'phantom levels'
I ran a game a few years ago, where we had a brand new player (to our table and RPGs in general) join our level 7 party. This was a similar situation to yours, as I had to get them tough enough not to get perma-dead by the first enemy they encountered, but they would have been overwhelmed if I had them make a 7th level character.
What I wish I had done in that situation, and what user T.L.D. has done (for previous editions) was give them the HP, Armor, and Weapons of a level 7 character, with the features of a level 1 character.
My suggestion to you is that they start at level 1, but have the HP that their character would have if they were the same level as your lowest leveled player at the table (or the level of the table if that's how you're doing things).
Then give them any mechanical bonuses commensurate to the party's level that do not increase the complexity of their character. This may include:
increased proficiency bonus
increased HP
better armor (magical if appropriate, limited to +1's and +2's)
better weapons (magical if appropriate, limited to +1's and +2's)
class features that do not induce choice or increase complexity (e.g. Monk's level 6 feature to overcome resistance to non-magical attacks)
Then, after each session (or two), they replace 1 phantom level with a real one. They do not get more health (you already gave that to them), but they add their class features, feats, spells, etc... that a character of their level would have.
This way, you don't have to be afraid to hit them, but they can still learn the character at a normal pace. Obviously, this works best for more martial classes vs casters.
Best Answer
It just doesn't matter
There is no 'best' here and the bounded accuracy of 5e really makes this fairly simple.
My group had a rotating-DM campaign that people could hop in and out of playing based on their availability. We all had different XP amounts and the levels spanned everything from a difference of 1 level to some who were 2 or 3 levels back or ahead.
And there really were no problems for any of us. I couldn't attend a lot of the sessions, so my bard was behind in XP and I was still able to contribute quite effectively (some, like my bard, might even say more effectively than the others.)
Don't worry about being a different level, just enjoy it.
There is one thing to possibly worry about
You may be fighting monsters who would normally be above your paygrade. Maybe. But you're in the group and have everyone's support. The monsters may hit harder than you expect, but there's always a mix of times when you're built to kill and others when you're not as effective regardless of your level.