Hi and welcome! Glad you found us, I also joined quite late into the game, Humphrey was 6 months and I so wished I‘d had access to all the wonderful help, advice and support on here in the beginning.
So, first and foremost, your little one (what‘s his name?) is quite normal and all of us who‘ve had our charges since they were pups have been through the same things
Good news is, they‘re all phases that he‘ll grow out of - eventually!
Teething and biting can be a real pain, for you and him! Kongs filled with yoghurt and popped into the freezer are great for soothing sore gums. I bought a wooden tree root chew for Humphrey, they‘re not cheap but last for a long long time, they‘re hard enough to chew on but not so hard as to cause damage to the teeth. I tried spraying the furniture with an anti chew spray which I bought at the pet shop but it wasn‘t very effective, the only sure way is to deny unsupervised access to anything of value. Its amazing how tidy the family became all of a sudden!
The garden problem was my nightmare! Again, it does get somewhat better but it is difficult to drive home the idea of borders vs. grass! We put up a cheap wire fence for the first summer so he had the run of part of the garden but I didn‘t have to watch every second.
Recall from the garden I solved with sheer bribery! Even now, at nearly 3, I‘m often to be heard calling „Biscuit“ as that‘s the only way he‘ll come in, although outside on walks his recall‘s good. You‘ll find he‘ll probably want to go out more often, just so he can come back in,
-they‘ve clever little so and so‘s but yoz can sort that problem out later!
I don‘t let Humphrey wander around in the garden on his own too much anyway, he goes out for a wee and a bit of a sniff but onl for a couple of minutes, - if they’re allowed too much „freedom“, especially when the’re little, I find they get a bit too independant. I don‘t think I‘d muzzle him, you could take him on a long lead and train a recall that way necessary but supervision is a better alternative really.
There‘ll be lots more advice coming your way I‘m sure. In meantime, good luck, keep us posted!