You want your retirement to be involuntary????? Retirement (pension/annuities) is a benefit provided by an employer (not all employers offer a pension program). As such, retirement (regular/early/and disability) is always voluntary. You are not forced to apply for disability retirement. Separation; however, can be voluntary or involuntary.
By waiting to submit your disability retirement application you are shooting yourself in the foot!! It could take as long as a year to a year and a half from when you submit your application until your case is finalized (and that's if it's approved on initial submission).
If you are in FERS you must also apply for Social Security Disability Insurance (SSDI). Only about 15% to 20% of those applying for SSDI are approved initially, and about another 5% to 10% are approved on appeal. There is generally a six month waiting period from approval date until you begin receiving payments; however, in the case of being on LWOP that waiting period is backdated to your last work date. You must provide OPM with proof that you applied for SSDI. If you are approved there is a 100% offset on your FERS annuity the first year and 60% the second year on.