Migration from Lync 2013 to SFB Server 2019

Team,
Our existing setup was,  Lync Server 2013 Enterprise Edition Pool ( 2 Front End servers in LyncPool, 2 Backend SQL Mirroring with SQL Witness Server and an OWA Server. Basic usage and no Internet in Network. Purely Internal Use only.

Hence, Migration to SFB 2019 was chosen over Microsoft Teams.
New setup, 6 Servers - 3 Front End Servers in SkypePool, 2 SQL Servers with Availability Group Listener, 1 Office Online Server.
1. Prepared Office Online Server

2. Completed initial Prerequisites like DNS, Server Prerequisites, Install Administrative Tools

3. On SFB 2019 FE Server, downloaded Existing Topology and add the Enterprise Edition pool of SFB 2019. Published successfully

4. Created Databases Manually because install-csdatabase failed, Setup SQL Availability Group and re-published topology successfully.

5. executed sql failover and republished topology with new server so that those security logins get permissions in second server.

6. Installed Front end role on all 3 servers successfully

7. Created a new user on SFB 2019 and tested. Later migrated one old user from Lync 2013 to SFB 2019 and tested. Login works correctly.

Now my queries, please help.

1. I'm unable to change CMS Server in Topology because it is greyed out and gives a message saying after installing you can change through Operations. confused here.

2. what should be my next step? shall I migrate all users to new pool or change CMS and make new URL's active? I'm lost at next action items.
I need directions in doing smooth transition and removing legacy servers safely.