You might try using a PHP version that is lower than the advised, only for the upgrade process. Immediately afterwards, move the PHP version up.
Another approach is to try to work with composer to solve that particular error. Check the Robo version that is required for the old and for the new SuiteCRM (that’s specified in composer.json). Then you can use composer upgrade (not the usual command which would be composer install - the names are misleading) to move the Robo version up before starting the upgrade.
Make a backup copy of the vendor directory before messing with composer, and don’t run composer as root.