Sending everything to your Gmail so your POP3 email can be handled by an IMAP service is a HUGE advantage and should help simplify handling spam a LOT. That way, you can setup spam filters in one place, one time, in Gmail. Email is spam-filtered on the server so it is sent to your devices, computer, wherever pre-sorted.
Since they are POP3 accounts, none of that is possible if handled in the usual way. Filtering spam must be done individually, on your phone or computer. IOW, if you separate the accounts, you'll have to do all that configuration or each account and on each device. None of the other advantages of IMAP would be available such as support for folders, syncing across devices, push notifications. etc etc.
Keep forwarding email and take advantage of all that Gmail and IMAP service can do for you.