Only employee contributions can be made to your post-tax Roth solo 401k. Any employer contributions must be pre-tax.
For 2022, the employee contribution limit is $20,500 ($27,000 if age 50+).
For 2023, the limit is $22,500 ($30,000 if age 50+).
You can max out your employee contributions as Roth, or you can choose to allocate some to Roth and the rest to pre-tax.