imac 2011 mid(27')issues: 1 beep every 5 sec! defective power board? dying GPU?

Dear iMac talents

A basket of questions to ask, please help:

1. initial problem is frequent auto restart of imac, even without prior use in the early morning.

*Hardware disgnostic report said no hardware trouble, but i know the report is not conclusive nor definitive.

*I had moved some doubtful windows MS and Adobe softwares to trash bin.

*Already reinstalled high sierra latest OS from internet.

*called Apple hotline, still no big progress.

*called local mac repair shops; technician claimed maybe a defective GPU, a defective hardisk, without inspection of my mac.

2.since this is obsolete model, no longer with parts or maintenance service from Apple. And it is extremely heavy, and i decided to open up the mac, check. Observations:

*heavy dusts in bottom of mac, where 4 ram cards are located

*white detachable powder is around border of power board, as shown in yellow-lined photo

*GPU covered with dry thermal paste, on logic board

*remove dusts by Dyson vacuum, brush. (did not have canned air. )

3.After assembly/reseat every parts, worser observations:

*panic kernel x2 times

*1 beep every 5 seconds; tried to plug 1 ram, 2 rams in top slot; and 4 rams in 4 slots(top + bottom). The Samsung RAMs were normal running before, but after this surgery, looks suddenly defective. photo was taken. correct type RAM was used. i will test if my ram is broken with other computer and report here.

Here are my stupid questions:

1. can the RAM slots be damaged? the rams and slots seem not tighly contacted with each other before i closed the bottom panel.
2. what is happening now?
3. what will you do?
4. is GPU, Ram slots, logic board, power board at fault? or every parts at fault?
5. if i open the mac internal again, can LED lights guide me what parts to check?

i feel very frustrated. i have not been employed for 2 years and hence am not ready to spend more until i can diagnose the mac problem(s).

And i bought this 2nd hand mac from an Indian couple half year ago. the mac has been performing quite good until recent 2-3 weeks.

please help, my mac god.