Replace with ASA with FTD - not that as expected easy task as per your experience.
But not that hard as it is, but couple of things you need to consider,
1. what features in ASA you are using ? is this configuration can be migrated to FTD ?
2. Cisco have ASA to FTD Migration tool (which does 80% of the work, again depends on the how ASA configured)
https://www.cisco.com/c/en/us/products/security/secure-firewall-migration-tool/index.html
3. Other side Upgrade the FTD with latest version,. and choose the right GUI , FDM vs FMC ( FDM on box, FMC out of the box Managment solution)
4. Physical cables and routing stuff. (If you like to use same IP then you need to test Offline and replace the device when you have maintenance and replace and test it)- if any issue role back to old ASA or fix and move forward depends on how critical this device in the network.
You can extract each steps more dependencies based on your requirement, make sure test is keen here before you live and all working.
Note : i will also clean up any FW ACL not required in ASA so you do not carry any Legacy non-used policies to new environment.
If you think this is big, then contact any integration engineer who can do small project and migrate for you, you can shadow and learn, so you can do yourself next time.
