Hello Japio342,

to link the first with a second and a third switch use a trunk port. Connect TL-SG105E port #5 with port #1 of the UniFi switch. Assign both ports a tagged membership of both VLANs 1 and 2. When using tagged frames only on a trunk port, the port's PVID doesn't matter. Do the same for the TL-SG1016DE.

@Japio342, to be honest, I do not understand your questions.

What do you mean with »how TP-Link is using [VLANs]«?

There is only one way to use 802.1Q VLANs: it's the way the 802.1Q standard defines and which every switch manufacturer needs to follow in order to have their switches classified as 802.1Q-compatible. The standard defines mechanisms only and the one who determines how a VLAN mechanism is to be used is the network admin.

Configuring a switch does not disrupt its services except if you change ports used for this service or if you reboot the switch.

Dear @Japio342,

Please ensure the connected ports among the pfsense router and 3 switches are set as turnk ports in corresponding VLANs. On TP-Link switches, select the port as Tagged port to achieve VLAN Trunk feature. For your reference, please check the configuration example of VLAN on TP-Link switches (the router doesn't support trunk VLAN in the configuration example).

https://www.tp-link.com/support/faq/788/

Hope the information is helpful.