Among other things, we have a mint 1967 Commander pop-up and are involved in 1-2 old camper groups/forums. Flex seal does not have a good track record in those groups. It works for a while (apparently just long enough to film a tv commercial...), then reportedly becomes a total abject failure and liability to adequately remove in hopes that a proper repair can be made.
My wife worked as a professional guide, running camps, lived in a tent for 4 years, etc etc hard core btdt doesnt screw around with such things. We have had our big Eureka Copper Canyon tent for nearly 20 yrs and love it. Dont waste a good tent. Fix it right. Use legit product like nikwax waterproofing or seam sealer as suggested.