Flitting between the past and present tense when writing?

It depends on how well you can pull it off. If you read a lot, you'll find novels where the writer adds a bit of present tense amongst all the past tense. It can work if you can write it well enough. The Radley's by Matt Haig is the most recent novel I've read that has a mix of present and past tense - I mean a lot of it.