You misread my previous post. I'm not suggesting that you loose anything:
archive the rest locally on your desktop client (also make a backup of the file)
What i'm suggesting is that for daily usage you probably wont need to search through mails that are older than a year or 2, and most of the emails that you received through this time are probably not important (newsletters, stuff already dealt with, ect).
So if you take 20 mins to browse through these you'll probably be able to sort out the few mails that are important (like bills, taxes, invoices...) and that you might need to search for later and then you can forward them to your iredmail account.
This way when you use the search function on your mobile phone you won't have to download 12 years worth of data and wait until your server has to browse through all of them to get your search result.
For the exceptional times when you need to search for an email that's older than 2 years you just do it on your computer through your local desktop client archive.
I have over 20 years worth of mail, i run a business so mail is very important to me and i've been working this way for years without any problem. So i don't see why you couldn't do the same
Your email is your personal data, and it should be easily transportable. I made the mistake of entrusting my data to a garbage company like Google, and I don't want to make that mistake again. Whoever or whatever I switch to needs to be able to both bulk import and export its data.
I already explained to you how to do it. It's actually easy if you're familiar with IT. For importing emails it's not a "1 click button import file" but if you want to manage your own mail server, whatever solution you choose won't be all about 1 clicking anyway. It will take you time to setup, time to get familiar with and time to upgrade/backup on a monthly basis.
So if you find that converting your mbox email database to a maildir format is too difficult, i suggest you pay a hosted secure mail solution that will be better for your privacy (like protonmail) and you dont have to deal with maintaining a mail server by yourself.
Otherwise get ready to invest some time into it and to get your hands dirty (well in a manner of speaking of course )