As written it seems the shape of the tumor can be changed each time it is detached from the alchemist's body:

the alchemist can have the tumor detach itself from his body as a separate creature vaguely resembling a kind of animal suitable for a familiar

There is no mention of the first time the tumor is detached being different from the other times, so each time it detaches it gets a shape.

However it seems likely to me that what was meant here was that you had to choose the shape of the tumor the first time it detaches. I think this mostly for a question of balance as making you able to change the tumor's shape at-will makes you able to get +3 to any skill when needed, among other strict benefits compared to a regular familiar.

1. Wasp Familiar:

This feat grants you a familiar using your character level as your level for it.

2. Skill Focus/Eldritch Heritage (Arcane Bloodline)

Detailed more closely in this answer, Eldritch Heritage (Arcane Bloodline) would increase the effective level of your familiar.

3. Iron Will/Familiar Bond/Improved Familiar Bond

I know you'd rather not go with this, but it is a way to get a familiar that progresses with your level.

Honorable Mention: Boon Companion

This feat does not give you a scaling familiar, however it does increases your familiars effective level by 4.