Can you wait until a higher level to make a Magical Secrets selection?

Question

I'm looking to build a an Eloquence Bard 17, Great Old One Warlock 3, Changling (Master of Disguise and Tongue)

If my character survives, and we play long enough, I believe I will get 4 Magical Secrets. However, I will not get the last ones at Bard 18th level (9th level spell slots)

Question: Can I wait to fill a known spell (magical secret) until I reach a higher spell level capability?

...Such as waiting until 20th level, when I finally get access to 9th level spells?

This basically means that I acquire one less known spell until I hit 20th level, which seems a fair cost to get it. But, not sure if I can wait to fill my secret learned. Roleplaying wise it could make sense to wait to find the secrets I most desire, but not sure about RAW.

Answer 1

No. You must select your spells when you get the Magical Secrets feature.

The bard's Magical Secrets feature at level 14 states:

At 14th level, you have plundered magical knowledge from a wide spectrum of disciplines. Choose two spells from any classes, including this one. A spell you choose must be of a level you can cast, as shown on the Bard table, or a cantrip.

The chosen spells count as bard spells for you and are included in the number in the Spells Known column of the Bard table.

When you reach 14th level, you have this class feature, because it says "at 14th level", and the selections must be made at that time, from the options available at that time, since nothing in the feature description indicates that it happens at any other time, only "at 14th level". There is no rule that says something like "you must take a class feature at the level it says you must take it" because the feature tells you at what level you have it.

Missing out on being able to select a 9th-level spell with Magical Secrets is just one of the tradeoffs of taking three levels in another class. If a 9th-level spell from a different class list is the secret you most desire, you shouldn't have a problem taking only two levels of warlock.