Local MacMail option for lareg IMAP message databases

I have some clients that have been using IMAP accounts (through Intermedia) for a very long time, and their storage usage is becoming a major cost issue. The hosting provider appears to be try to push everyone to MS365, so they progressively ramped up the pricing for storage to the point where it is unworkable. The users would prefer not to switch to MS365 (and pay the higher monthly), but their IMAP storage costs are rendering the $2.85 / mo / user pricing as meaningless savings. So, I was hoping to take several years of past email history and make it locally availability on their MacBooks and iMacs. What would be the best way to do this? I can export the whole history as a .PST file, but this has its limitations. I want to be sure the local "old" repository is easily accessible and safe (backed up) on their machines.