Learning to Navigate DNS records

Noobie here.

Right I've got some websites hosted through IONOS. .I also have the email packages as it's so cheap.

I'm having a NIGHTMARE with them.

I've had to wipe one websites DNS records as iCloud messed it up, which meant re doing some of the formspree TXT records which was a pain.

Question 1: is thunderbird a good client? All my emails seem to land in Gmail or outlook before hitting thunderbird inbox.

Question 2: is IONOS hosting any good, seems costly to move over to Google or microft but I've read they tend to be much better. (My emails keep landing in customers spam)

Question 3: Email signature, I created quite a fancy signature. About 70kb I think. Is that a bit daft ?

Question 4: why is one of my domain emails asking me to manually configure when I have the autolearn record applies ?

TIA