Migration help!! Stuck on 2.1.27.36

[Cheechy]

Hi,
We are desperate for some help :)

Teampass 2.x.x.36 is stuck on an old debian jessie version on php 5.x
We have a new debian bookworm server running php 8.x

We do not appear to have any migration or upgrade option :(

Copying the files and db to the new debian doesn't appear to work to upgrade.
ie - php8 won't run 2.x version
Tried many different 3.x versions, the upgrade does not get past the first screen. IE, after importing the DB (which has also gone from mysql 5.x to latest), the admin password does not appear to work
error is: PHP Fatal error: Uncaught Error: Undefined constant "DB_PASSWD" in /var/www/teampass/sources/main.functions.php:4297\nStack trace:\n#0 /var/www/teampass/sources/main.functions.php(61): loadClasses()\n#1 /var/www/teampass/install/upgrade_ajax.php(29): require_once('...')\n#2 {main}\n thrown in /var/www/teampass/sources/main.functions.php on line 4297,

Simply can't get an upgrade working after exporting/importing the DB across servers and trying to start it off

On the existing server, an in-place upgrade does not seem possible?
Teampass 3.x requires php7.4
I am only aware of a couple of solutions to upgrade php on debian 8, which is via dotdeb.org (Which only appears to have 7.0 and also the gd module fails to compile) OR https://github.com/kasparsd/php-7-debian which fails to finish the build these days.
Possibly other options at a big cost like freexiom...

What do we do ?
I can't even get the latest 3.1.2 to install on latest debian (install screen doesn't move past first screen). I had to go with 3.1.1 which installs without an issue on same server ???

We would love to upgrade Teampass but there seems to be no option for us. Even latest code doesn't install fresh.
Any advice or help very welcome!