I paid 4000 to get my tourist visa extended, without an agent 1900. Doing it myself required me to make 3 freaking trips to the visa office, 1) didn't have the right paper, 2) can only extend 3 days before expire, 3) ....
Using an agent makes immigration freaking easy... Its not always about being illegitimate. Sometimes its about convenience.
There is a company (the owner of this group) is offering to do your Thai Pass + insurance for 1900 baht. That is what I would do if I were in your position.
You will probably be ok. One concern is the overstay and any future business you may have with Thai immigration. If you plan to visit only once in your life then you're fine. If you think their maybe a possibility of return trips I would try for the visa or get an extension while your are here (visa is better option).
I never update those things. If you have the receipt and don't need to go to another immigration office, don't bother updating it.
If I get my TM30 out of pattaya then go to phuket for a few weeks, return to pattaya to exend my visa, I dont bother updating. If I extend out of Phuket I will update.
I have been pulled aside a couple of times. They yelled at me for a few minutes then let me go on my way. When I was in BKK airport, getting yelled at, I watch them pull-over some other gentlemen and yell at him. His paperwork was in order but it was in the senior citizen lane and his wife when through the normal lane, she had all the paperwork.
I am convinced they randomly select people and yell at them for a little bit. If you're not a dick they'll send you on your way.
Thailand, pre-covid, was unlimited in visitor visa and visa exempt (by air). The policies are generous especially if you have financial means.
Did you look to see the cost of AXA for 29 days? When I was shopping for insurance AXA seem to be monthly prices. When I set the dates for 6 weeks was the same price as 8 weeks.