Should I keep the old pump/filter set and get another 'small' filter/
pump set (and use both at once), or scrap the old and get one larger
new set?

Keep the old, but get a new larger one. The old is already cycled...
assuming you're going to some how keep it running during the remodel?
Perhaps on the holding container? ~ jan
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Zone 7a, SE Washington State
Ponds:www.jjspond.us

Well, that's a question too: I've got a clean 55 gallon HDPE drum I
could put the creatures in during the remodel. How long do you think
they would last in there? It would be awfully tight. And the frog
won't be happy without a view...