A 3500 watt element adds only 11,946 BTUs/hour. Inlet temperature at 40 and outlet temperature at 110 and 2.5 gpm = 87,465 BTUs/hour. So you are using it at a rate almost 8 times faster than you you can make it!
If WH temperature is set to 110F you would use no cold at all and get 10 minutes if you have 25 gallons available (as opposed to the 20 Randy suggests, but Randy is probably right). If you could set the thermostat to 180F (most don’t go that high) you would mix 1.25 gallons of cold with 1.25 gallons of hot for the same outlet temperature of 110F and then get a 20 minute shower.
But scalding is a real possibility at this temperature. A mixing valve would be good here to mix the hot and cold at the WH so to prevent scalding.
Even two 3500 watt elements will not give you more than an extra minute. You will have to have adequate BTUs stored to start with. Either more volume (larger tank), higher temperature, or both.