Architecture changes when migrating from SharePoint On-prem to Online

[KVollero2000]

I'm the farm admin for a 300 gigabyte SharePoint on-prem application. This is a legacy site for a state agency, having started as SharePoint 2003 and been upgraded over the years all the way to SP2019. Am considering the move to SharePoint Online, but the app has deeply nested sub-sites, some going as far as 8 levels down. We have a custom web part which provides menuing from top to bottom. In the new Online world, the architecture is now flat. I'm told that going any more than 3 levels down is a concern.

Where would I go for guidance on learning how to migrating / converting from such deeply nested sub-sites to a flatter site architecture? I've seen very little on how to deal with this problem.

Thank you,

[#2]

Well, everything should start with some planning & design where you define the number of Hubs and Sites you are going to need so you can provision them before start thinking on any migration...once you are done with that, you can start planning the migration where you will need to map your subistes with the new sites in SPO

[#3]

Yes, that's what I'm asking about. We have a large SharePoint collection, with about 5,000 individual sites, some nested as many as eight levels deep. I can't even conceptualize how to flatten that out and still make it navigable in an intuitive way. Planning/Designing starts with "This is where we are" and then you make a plan to get to "This is where we want to be". I'm having trouble figuring what the target should look like.

For background, I've done SharePoint farm upgrades, alone and with no assistance, from 2007 thru each iteration up to 2019. I know how to migrate and convert content, even with custom web parts. I just have no idea what the Online version of my current farm would look like.