You don't understand what I'm saying.
You want to use an expansion feature.
Feature 1: DH's.
How to get them?
1. Use boost you got with your Legion purchase.
2. Bam, you got your DH unlocked.
Cost: The time you need to level from 98 to 100 (since your boost boosts to 100 and DH's start at level 98, so you've lost 2 levels in this process). The time to level from 100 to 110.
Feature 2: An allied race. Lets take Vulpera as an example.
How to get them?
1. Use your boost to 110. (Why would you count the SL Boost? Allied races are a BfA feature, so I have to use the BfA boost. For DH's the Legion boost was sufficient. Also Shadowlands base game does not provide you with a 120 boost)
2. Level from 110 to 120
3. Do WQ's for a month
4. Bam, Vulpera unlocked.
Cost: 1 month of grinding rep (since your Vulpera won't have access to the rep you've earned). The time you need to level from 20 to 120 (because your Vulpera starts at 20). The time you need to level form 110 to 120.
So yeah. The effort to unlock one (!) Allied race is much higher than the cost of unlocking DH's.
You know why Legion was much more successful than BfA is? Because a new or returning player could just buy the base game, and play everything the expansion offered. BfA is quite the opposite of that.