Migrating: Single Router by One Manuf. to a Mesh by another- - Will the Old Credentials still work?

I've had the same Asus RT AC 68w for 5 years and I've added a large mix of IoT Wall Switches / Outlets, and the count is likely 50+ Devices. 1 1/2 years ago I thought it would be an easy upgrade to add another Asus 68W to Create a MESH.... That's where the easy stopped. Long story short.... Two (2) Bad 68W's in a row... I then tried two other more expensive Asus Routers.... And quickly learned throwing money at this issue was the wrong approach. Each additional Router I tried to get to MESH failed in all kinds of ways. Asus could not resolve the problems and I just gave up at that point and kept my old Router.. hoping one day the Mesh market would mature. So fast forward to today.... a friend bought the TP LInk Deco M9 Plus and his son spent 3 hours configuring it, but it works he claims and has no dropouts... Just about any Mesh System (or Router) has a long list of unhappy customers when reading the reviews, and I know some issues are self inflicted.....

So I just received my Deco M9 Plus 3 pack, and out of the gate, issues! First it would not accept my password.. I used the same Name & Password to hopefully not have to RESET every WiFi Device I own..

After numerous "chats" with TP Link (the agents I spoke with definitely need better tech training on there products) they asked me to forget about the 2.5 /5g, and setup a GUEST account with a different password. In frustration I did so, but only added one letter to my 10 alpha/numeric password. The 11 alpha/numeric worked, but why? I asked what are the DECO minimum / maximum requirements for Passwords? Of course... They did not know..... I then started to RESET all my devices, and I started with my 6 Alexa devices. All worked but one..the Echo Show would just not connect! I then took the WiFi system off line, did a Factory Reset of all three, and started fresh, this time I only hooked up ONE Deco to Test..... This time NONE of my devices would connect. My 3rd party wifi analyzer showed the signal... but would allow no connections.

Chatted again with the help team and another round of "I've already answered your same questions Twice" before they would escalate the call, and then told me they would have to forward it to the engineers for follow up.....

So really... my question is: If (and when) I get a working MESH system, should not all of my devices readily connect to the new Wi-Fi if I use the same Name & Password credentials they were set up with?

Do devices look at more than the Name & Password? Do I really need to RESET every device I have when I introduce a new WiFi Router?

Thanks for any suggestions... I've already worn out one paperclip resetting those Mesh devices!