Since you seem set on keeping the iPad and not putting in a real EFIS (it’s your airplane!) I’d suggest the following as the low cost solution:
Keeping your iPad and ADSB-in set up.
Buy an gtx327 (mode C) transponder, available used almost anywhere for $500. Make sure it fits in the space you have below the radio.
Buy a compatible altitude encoder, $300 (new).
Buy (from GRT) a combo uAvionics Echo UAT with a GRT Safefly gps, $1500. This can be remote mounted.
Two antennas (transponder, UAT), $100-$300 each. Total $2.5 - $2.9K
Option B.
Check if the control head for a Trig TT22 will fit (1.8” high). If so, then
Buy trig TT22 mode S-ES transponder (control head on panel, electronics remote mount), $2.2K
Buy encoder ($300)
Buy GRT safe fly gps ($600) or Trig gps ($500).
One antenna $100-$300. Total 3.1 - $3.4K