If you do them yourself keratin treatments are not that much more expensive compared to a relaxer IMO.

I would recommend a non permanent smoothing products like design essentials strengthening therapy,naked assuage thermal smoothing complex,arosci intensive restructuring or huetiful nano smoothing treatment. these are mild and definitely do not cause a permanent change so you can get the loosened look and have the option of your hair reverting back.

a step up from that would be a keratin treatment which are more plentiful and cheaper but tent to be a crap shoot in terms of over or under effectiveness of the specific formula.

or you can go the permanent rout which is the relaxer/ Japanese straightening. these last longer than the others but you will have to upkeep with the new growth after a while.

In the end it depends on the look you want to achieve and the time and money you are willing to spend on your hair.
4a transitioning
Straight haired natural journey