I was in a similar position and I bought a reciprocating saw. It was OK, but very slow and really struggled on anything bigger than about 12cm.
Then a bought a Bosch electric chainsaw. It takes about a quarter of the time, will cut up to around 30cm and I’m on the original chain still. I used to go through reciprocating blades like they were going out of fashion and they are not cheap. You still have to be careful with electric saws, but they have good kickback brakes and the chain stops immediately if they kickback or you take your hand of the trigger. Use PPE, be sensible and work at the right height and you’ll be fine.
The things you have linked to above are for occasional light use only – pruning etc. No good for what you want.
oh, and I made a saw horse from some 4×2. It took about half an hour.