Worked outside visa's scope; immigration advice?

Throwaway account. I searched for this topic, but only found one from 7 years ago.

I worked last year (2025) part-time while on an instructor visa in Japan as my employer then told me my visa would be OK to do so with them.

I realized that was a lie when I mentioned it to a friend a few months ago & I quit that part-time job shortly after & found a job that’ll sponsor my visa (still same instructor visa) & now work for them.

Problem is when I went to immigration to renew my visa, they wanted more documents, like tax payment forms (kazei-shomeisho & nozei-shomeosho) or a tax exemption form (hikazei) if needed for the months when I wasn't full-time working (at the part-time) from last year until now.

Immigration will do the math & find the part-time work on the wrong visa for about half a year (I think I forgot to put that on my visa renewal form, but I'm sure the part-time job put me down as an employee on their tax records while I was with them) & likely didn't make enough to get taxed properly while at the part-time.

I plan to submit the forms immigration wants before their deadline, but when I asked immigration lawyers (aka scriveners) how badly the penalty will be, they didn't know & can't help once the visa application has been submitted to immigration (it has). I called immigration anonymously to ask what to do in this situation & they said to fix any mistakes on my renewal forms & submit the tax documents & an explanation.

What I want to know is this:

I'm 99% sure I'm out of luck & likely lose my visa & get a re-entry ban for 1-5 years. My question is how bad is the legal fallout for this? How bad are the stories about Americans who worked outside the scope of their visa for as many months as this? Ever heard of stories where they got locked up (up to 3 yrs) & had to pay a fine (up to ¥3,000,000 aka $19K) as punishment for being outside their visa?

I know Japan has a hate-on for foreigners now, but the advice gaijin friends said is to comply, be honest, be genuinely apologetic & even write an explanation & apology letter to immigration & hope for the best. And if it’s denied, simply comply & prepare to leave & hope it's not an overtly severe punishment.

However, I want to brace for the worst (jail time, high fine, perma-ban, etc.) once the forms are submitted & immigration starts asking or just decides then not to approve renewal & maybe send immigration to my door or send me a notice to come down to their office.

Also, is there anything that can be done to lessen the fallout? Can I self-deport at immigration & pay back taxes? Can I do it before my current visa expires? Is there anything else that can be done to mitigate the penalty from this? I've no criminal record in my years in Japan, but don't know what else is a factor in how immigration decides to punish foreigners being outside their work visa scope & behind in taxes other than being very harsh (from what legal sources had told me).

Should I contact the part-time job & explain my situation to them & see if they can help with the paperwork? Or is it better I do it on my own & confirm with immigration who the part-time company was? Is there any other gaijin-friendly help I can turn to for a situation like this?

I've more questions,  but this is it for now. Thanks for any clarification.