For Slice reading , you can using load_uint(8) in FunC.

More detail can check with example like here:
https://docs.ton.org/develop/smart-contracts/guidelines/tips#spend-less-gas-on-large-smart-contracts

() recv_external(slice in_msg) impure {
     var signature = in_msg~load_bits(512);
     var cs = in_msg;
     var (subwallet_id, valid_until, msg_seqno) = (cs~load_uint(32), cs~load_uint(32), cs~load_uint(32));
     throw_if(36, valid_until <= now());
     var ds = get_data().begin_parse();
     var (stored_seqno, stored_subwallet, public_key, plugins) = (ds~load_uint(32), ds~load_uint(32), ds~load_uint(256), ds~load_dict());
     ds.end_parse();
     throw_unless(33, msg_seqno == stored_seqno);
     throw_unless(34, subwallet_id == stored_subwallet);
     throw_unless(35, check_signature(slice_hash(in_msg), signature, public_key));
     accept_message();
     set_data(begin_cell()
       .store_uint(stored_seqno + 1, 32)
       .store_uint(stored_subwallet, 32)
       .store_uint(public_key, 256)
       .store_dict(plugins)
       .end_cell());
     commit();
     cs~touch();
     int op = cs~load_uint(8);